Formal speech and writing is sometimes different from informal speech and writing. In English there are certain words and structures which are mostly used in formal situations. There are also certain words and structures for informal situations.

Use of Contraction

Contracted auxiliary verbs and negatives are common in informal speech and writing. They are not normally used in formal situations.

He has gone. (Formal)

He's gone. (Informal)

I am ready. (Formal)

I'm ready. (Informal)

Use of prepositions

Prepositions can come at the end of certain structures in informal language. This is not possible in formal language.

Which nation does she belong to? (Informal)

To which nation does she belong? (Formal)

Use of relative pronouns

In informal speech, the relative pronoun can be dropped when it is the object of the clause.

The woman who you are talking about is my boss. (Formal)

The woman you are talking about is my boss. (Informal)

The movie which I saw yesterday was really nice. (Formal)

The movie I saw yesterday was really nice. (Informal)

You can usually decide whether a relative pronoun is an object because it is normally followed by another subject + verb.

Use of determiners

Some determiners are followed by singular verbs in formal language and plural verbs in informal language.

Neither of the answers is correct. (Formal)

Neither of the answers are correct. (Informal)

Informal use of object forms

In informal English, we use object forms not only as the objects of verbs and prepositions, but also in most other cases where the words do not come before the verbs as their subjects. Object forms are common, for example in one-word answers and after be.

‘Who said that?’ ‘(It was) him.’ (Informal)

‘Who’s that?’ ‘It’s me.’ (Informal)

In a more formal style, we often use subject form + verb.

‘Who said that?’ ‘He did.’

It is possible to use a subject form after be, but this is extremely formal, and is usually considered over-correct.

It is I (Very formal)

It is me. (Informal)

It is he. (Very formal)

It is him. (Informal)

Whom in questions

Whom is not often used in informal English. We prefer to use who as an object, especially in questions.

Who did they arrest?

Who did you go with?

We use whom in a more formal style; and we must use whom after a preposition.

Whom did they arrest? (Formal)

With whom did you go? (Very formal)

Ellipsis

Ellipsis (leaving out words) is more common in informal language.

Compare:

Have you seen Mr John? (Formal)

Seen John? (Informal)

We think that it is possible. (Formal)

We think it’s possible. (Informal)