Semantics is the line of study that is concerned with deriving meaning that is found in utterances. This involves several concepts including sentence relations to which presupposition and entailment belong. This category involves the following concepts; tautology, synonymy and contradiction. This paper serves to shed light on understanding of sentence relation, paying particular attention to presupposition and entailment, and the differences between these concepts to the effect of trying to clarify the confusion existent in there between.

Words do exhibit relations with one another in a language, sentences also do exhibit relations to one another. This means that sentences also relate in a specific way. You will find that words have relation that cause them to have similar meaning, also with sentences, they have specific relations of which such relations may be led by the reason of the use of particular words. Sometimes these relations may be of certain syntactic structure. But for one to know or be familiar with sentence relations, he or she must understand different concepts that fall under sentence relations. These are; synonymy, tautology, contradiction, entailment and presupposition. Below will be the description of each concept, basing more focus on entailment and presupposition.
Synonymy is a concept that deals with two sentences having the same truth value. Synonymy also can be confused as entailment. This is because when tested using the entailment concept, synonymy tends to hold. Two sentences p and q are synonymous if they both have the same truth value; that is if p describes a situation, q will also describe the same situation. Such sentences are referred to as paraphrases.

Jones owns the house
The house belongs to Jones
The composite truth table for the synonymous sentences above is as follows:
P q
T > T
F < F
T ? T
F ? F
The table is also applicable to 3 and 4 below. However the synonymous relation in this pair is induced by the syntax of the sentences.
The police pursued the robber
The robber was pursued by the police

Two sentences p and q are contradictory if they both affirm and deny the same proposition. For such sentences, when p is T q is F as shown below:
He is a bachelor
He is married

If ‘he’ is constant in 1 and 2, then the two sentences are contradictory. The reason why these two sentences are said to be contrary is because they hold different ideas. If one out of the blue, without having the subject in place, uses ‘he’ without the referent, the sentences in place will have a contradictory meaning. For these two sentences not be contradictory, there should be a referent. An example will be:
Tom is a bachelor
He is a bachelor
With the above sentences, we cannot say they are contradictory because both of them hold the same truth value. ‘He’ in sentence 4 will be a reference of Tom.
Huang defines presupposition as ‘a proposition or an inference whose validity is taken for granted for a sentence to be true or for a speech act to be felicitous’, (Huang, 2007). Presupposition constitute at any given time the communicative background assumed to be known and accepted (Franck 1973). In most cases, people use presuppositions throughout without knowing it. For one to understand presupposition, first they must know the definition of what presupposition is, the types of presuppositions, what they do and how one can identify them. Also, one must be able to know what causes presuppositions, which is referred to as presupposition triggers.
Kroeger defines presupposition as ‘information that which is linguistically encoded as being part of the common ground at the time of utterance’, (Kroeger 2018). With common ground, Kroeger refers to everything that both the speaker and the hearer know or believe, also know that they have in common. This means that during the time of utterance, the hearer has the knowledge of what is being said by the speaker. Examples that are obvious include; knowledge about the world, we all know that there is one moon and one sun. This is a fact that no one can dispute or argue because evidence is already there. This may also continue to include knowledge that is observable in speech situations, the perfect scenario will be what the speaker is wearing or carrying. Also facts that have been mentioned before in that same conversation or discourse. From the above, we can simply say, a sentence is said to presuppose another sentence if its truth and that of its negation both imply that the presupposed sentence is also true.
It is surprising that Magang passed the test.

Presupposition: Magang passed the test.

It is not surprising that Magang passed the test.

Presupposition: Magang passed the test.

The table below shows how presupposition relate or the structure in which it follows.

P q
T > T
F < T
T or F ? T
P presupposes q if q is part of the assumed background against which P is said. This simply means that q hold the same idea as P.

Under the concept of presupposition, there are different types that are found here. These types of presuppositions account for the difference found in presuppositions. They depend more on the context used or that is found in presuppositions. Under types of presuppositions, I will discuss six different types of presuppositions and give as much examples to elaborate the context.
This type of presupposition is the assumption that there is existence of entities that are named by the speaker. Existential presuppositions are the main starting point for presupposition theory philosophy (Seuren 1994, cited in Lamarque 1997, 359). What this means is that the speaker won’t speak of something that does not exist. Also, when asked to account, it will be easy because the speaker can pinpoint the entities that he is referring to.

Examples below will show how existential presupposition is like.

Mary’s new car is beautiful.

There is exist a person called Mary.

Mary has a car.

Tom’s cat is cute.

Tom exist.

He (Tom) has a cat.

Professor Otlogetswe’s car is fast.

Professor Otlogetswe exist.

He has a car.

From the above examples, we can see that what the speaker utters, the listener already know that there are entities that are being referred to. It is not something that is uttered out of the blue.

Factive presupposition is the assumption that argues that something is true, this ca be realized due to the presence of some verbs that are used in such presuppositions. To account for this, verbs such as “know”, “realize” and phrases that involve “glad” are used to show this type of presupposition. In most cases the presupposition is telling of what has already happened. The term ‘factive’ is used to classify these verbs. Examples that follow below will show how these presuppositions are realized.
He didn’t realize she was ill.

(;; she was ill)
Mary didn’t know that Thabo passed the exam.

(>> Thabo passed the exam.)
I am glad it’s over.

(;; It’s over.)
From the above examples, we can see that the sentences provided presupposes what follows after them.

With lexical presupposition, the assumption is that, with the use of one word, the hearer will understand the meaning that the speaker is relaying. The speaker act as if another meaning is obvious to the listener. The listener knows of the facts being stressed here. Examples below will help understand what exactly lexical presupposition is.

Thabo stopped smoking.

(>> Thabo smoked before) the implication is that Thabo used to smoke.

You drove the car again.

(>> you drove the car before.)
Mpho closed the door again.

(>> Mpho closed the door before) the implication here is that Mpho closed the door before, then he found it open and closed it again. It is something he did before.

A presupposition trigger is a construction or item that signals the existence of presupposition in an utterance. Such structures include:
Definite expressions:
p: John saw the man with two heads
q: There is a man with two heads
Cleft sentences or pseudo cleft sentence:
p: It was Henry that visited Rome
q: Someone visited Rome
p: What John lost was his notebook
q: John lost something
Lexical triggers such as factive verbs:
P: Mary realized that she was in debt
q: Mary was in debt
p: They both regretted eating the banana
q: They ate the banana
Change of state verbs
p: Have you started exercising regularly
q: you didn’t exercise
In pragmatics, entailment can be defined as the relationship that is found between two sentences, where the truth of one sentence requires the truth on another sentence. Saeed defines entailment as “a truth relation between sentences which holds regardless of the empirical truth of the sentences (Saeed 1997: 90). The truth of sentence B requires the truth of sentence A. This means that entailment can be referred to as a relationship that is found between two propositions that are expressed by two sentences.

Matome is an intelligent student.

Ent: Matome is a student.

From the above sentence, we can see that the sentence “Matome is student” in its preceding sentence.
Raymond and Lorato flunked.

Ent: Lorato flunked.

Lorato or Raymond flunked.

Ent: Someone flunked.

Mary broke the window.

Ent: The window broke
Tumelo and Ditiro went to the party.

Ent: Ditiro went to the party.

Ent: Tumelo went to the party.

Thabo is a bachelor.

Ent: Thabo is married.

F<T or F
T or F?T
From the above sentences, one can note that there are sentences that are entailed on the other sentences. The “Thabo is a bachelor” entails “he is a bachelor” when the pronoun ‘he” refers to ‘Thabo’. To the hearer, this will make sense because they already know that a bachelor the speaker is referring to is Thabo.

Presupposition and entailment differ. When a presupposing sentence is negated, the presupposition survives as in examples 1 and 2:
I regret studying law
Pres: I studied law
I do not regret studying law
Pres: I studied law

However, in entailment, negating the entailing sentence results in the failure of the entailment as in the following examples:
Peter is my brother.

I have a brother (entailment holds)
Peter is not my brother

I have a brother (we are not sure whether I have a brother or not hence entailment no longer holds)
Between sentence 4 and 5, there is no entailment because sentence 5 cannot be said to be entailed in sentence 4.

Presupposition also holds in questioning, embedding with modals and in conditional clauses:
The king of France is bald.

Pres: There is a King of France.

Is the king of France bald?
Pres: There is King of France.

The King of France might be bald.

Pres: There is a King of France.

If the King of France is bald, then he should wear a hat.

Pres: There is a King of France.