English seems a pretty simple language at one glance but when you go deeper into it you will realize that there are many terms and things in this language which you are not familiar with. Antecedent is one such term. It is an English grammar based word which has some context that we need to understand in order to use it in the right way.
If you are familiar with nouns and pronouns, it will be very easy for you to get the grasp of antecedent because it is based on these two parts of sentence. A noun is a name of a thing where as a pronoun is a word that is used in place of a noun. An antecedent on the other hand is the noun in a sentence which is represented by a certain pronoun in the same sentence.
Sara is a lovely girl.
She is a lovely girl.
Consider the above three examples. In the first example, Sara is used which is a noun. The second sentence makes use of only pronoun she but we don’t know which she the sentence is referring to. The third example is that of an antecedent which uses both noun and a pronoun i.e. Sara and she. In this sentence, Sara is the antecedent.
In the above example, a demonstrative antecedent is used. Can you figure out where? Yes, you got that right. Bus in the first sentence is antecedent which is replaced by this (descriptive pronoun) in the following sentence.