Affix

An affix is a bound morpheme which is grammatical in nature. There are two kinds of affixes – prefixes and suffixes. A prefix precedes the stem it is attached to, like the un- of unpleasant. A suffix follows its stem, like the –ness in kindness. The addition of an affix to a word is called affixation.

Agreement

In English the form of a particular word or phrase is determined by the form of another word or phrase which is grammatically linked with it. For example, the English grammatical rules require that we should use a singular noun after this and a plural noun after these. Similarly we use is after a singular subject and are after a plural subject.  This grammatical phenomenon is called agreement or concord.

Antecedent

An antecedent is a noun expression which provides the interpretation for a second expression. Note that we call the second expression an anaphor.
In the following examples, the noun phrases given in bold text are examples of antecedents, whereas the noun phrases given in italics refer to the corresponding anaphors.

If you see John, ask him to give me a call. (Antecedent – John; anaphor – him)
All of my students have learned their lessons. (Antecedent – students; anaphor – their)
Alice rang to tell me she would be late. (Antecedent – Alice; anaphor – she)

In some cases an antecedent follows its anaphor.

If you see her, give Mary my love. (Anaphor – her; antecedent – Mary)

Appositive

The label appositive refers to a noun phrase which immediately follows another noun phrase of identical reference. In writing, an appositive is usually set off by commas. The phrases set off by commas in the following sentences are examples of appositives.

John, my brother, is a cardiologist.
Mexico City, the largest city in the Americas, is highly polluted.

In most cases it is possible to remove the appositive without affecting the meaning of the sentence. For example, the sentence ‘Mexico City is highly polluted’ is both grammatically correct and sensible.

Some appositives are restrictive which means that they are required for the identification of the first noun phrase. A restrictive appositive is not set off by commas and it cannot be removed without affecting the meaning of the sentence.

An example of restrictive appositive is Tagore in the sentence ‘I am reading the biography of poet Tagore’. Here removing Tagore would affect the meaning of the sentence.