Idiom

An idiom is a fixed expression the meaning of which is not guessable from the meanings of the words in it. Examples include ‘kick the bucket’ (die) and ‘let the cat out of a bag’ (reveal a secret).
Imperative

The sentence type illustrated by the following distinctive sentence pattern: Come here! An imperative sentence commonly expresses a command. In English, an imperative sentence usually has no expressed subject though you is understood as its subject. In an imperative sentence, the verb will be in its infinitive form. In writing, an imperative sentence usually ends with an exclamation mark.

Examples are:

Wash your hands!

Sit down!

Get out!

Shut up!

In English, it is possible to use an imperative sentence to express an idea other than a command. For example, if you are going out on a picnic, I may say ‘Enjoy the picnic’, but this is not an order, only a hope.

Similarly, it is possible to use another sentence type to give an order. For example, the sentence ‘I order you to leave this place at once’ is certainly an order but it has the form of a declarative sentence.

Impersonal

A general label for any structure in which the action described is assigned to no one in particular. In English, the dummy pronoun ‘it’ is often used for this purpose.
It is raining.

It seems that we have a problem.

Other pronouns like ‘they’, ‘you’ and ‘one’ also have impersonal uses.

One must be very careful.

You can’t trust anyone these days.

Indefinite

A general term applied to all linguistic forms which refer to nobody and nothing in particular. Examples include the indefinite article a/an and indefinite pronouns like something, somebody, anybody, nobody, someone, anyone, somewhere, somehow etc.

Indefinite article

The conventional name for the English determiner a/an. The indefinite article does not refer to anybody or anything in particular.
I am looking for a job.

A spider has eight legs.
Indefinite pronoun

A pronoun which denotes nobody in particular is called an indefinite pronoun. The English indefinite pronouns are: somebody, something, anybody, anything and anyone.

The negative pronouns nobody, no one and nothing are also considered as indefinite pronouns.