We often use if to introduce possible or impossible situations or conditions and their results. The situations or conditions can be real, imagined or uncertain:

I usually make a sandwich to take to work if I have enough time. (real)

If you don’t book now, you won’t get good tickets. (real)

They’d have got the job done quicker if they’d had more people working on it. (imagined)

Will you bring my glasses down if you go upstairs? (uncertain)

We can sometimes leave words out after if to form fixed expressions:

Check the temperature of your meat with a meat thermometer if possible. (if it’s possible or if that’s possible)

Interest rates would have to rise if necessary to protect the pound, Mr John Smith, Shadow Chancellor, indicated yesterday on BBC TV’s Money Programme.

We use so or not after if when it is obvious what we are referring to:

[from a job advertisement]

Are you looking for part-time work? Do you want to work from home? If so, read on. (if you are looking for part-time work or if you want to work from home)

You should all have received your booklist for the course by now. If not, please email the office. (if you haven’t received your booklist for the course by now)

I’ll see you soon, definitely at the wedding, if not before. (if I don’t see you before the wedding)

We can use even if to mean if when talking about surprising or extreme situations:

You’re still going to be cold even if you put on two or three jumpers.

We use if to introduce reported yes-no questions and questions with or.

We use only if to express a strong condition, often an order or command, to mean ‘on the condition that’. It has an opposite meaning to ‘except if’:

Payment will be made only if the work is completed on time.

Alright I’ll come but only if I can bring a friend with me.

We often separate only and if, using only in the main clause:

He’ll only take the job if they offer him more money.

We’ll only achieve our targets if everyone works together.

In speaking, we often use if to introduce a polite request. If is usually followed by modal verbs will, would, can or could when it is used to be polite:

If you’ll just tell Julie that her next client is here. (Can you tell Julie that …)

If you would like to follow me. (Please follow me.)