Definition:

These sentences comprise two parts – the main clause and the conditional clause or the ‘if clause’.

They are used to denote that the action in the main clause can only take place if a certain condition (conditional clause) is fulfilled.
Some examples (the part in BOLD indicates the conditional clause or the if clause) –
1.     I would give you a chocolate if you got good marks in your exams.
2.     When a dog barks nearby her, Sakshi gets scared.
3.     Unless Anmol helps Nitesh, he will not be able to complete his work in time.

The following words/phrases are used to denote an ‘if-clause’ –

If; unless; as soon as; provided; no sooner; until; when; in case; but for; so long as; etc.

Some basic rules –
1. If the ‘if clause’ is in present tense –
If + Simple present, Simple future

Example – If Harish gets a wicket on this ball, our team will win the match.

2. If the ‘if clause’ is in simple past tense –
If + Simple past, subject + would + V1

Example – If I had money, I would buy you those shoes.

3. If the ‘if clause’ is in past perfect tense –
If + past perfect, subject + would + have + V3

Example – If you had told me about this earlier, I would have helped you.

These sentences can also be expressed/written according to the following structure without any altering to their meanings –
Had + subject +V3 + Object, Subject + would + have + V3

Example – Had you told me about this earlier, I would have helped you.

Some important points to remember –
> If a probability is expressed in a sentence, ‘may/might’ are used instead of ‘will’. (Same for the sentences about permissions)
Examples –
If it rains, I may not go to office today.
If you have finished your home work, you may go out to play.

> If the sentence is related to suggestions, ‘should/must’ will be used instead of ‘will’.
Examples –
If you don’t know her, you must not go out with her.
If you want to score good marks, you should study daily.