if I were you - if I was you - which is correct?

Conditional sentences (if-clauses, main clauses)

 

The word were in the phrase if I were you is special form. It is known as the subjunctive mood (from the grammatical point of view).

Today you also find the phrase if I was you. Here the Simple Past form of be is used. But there are people who say that this phrase is incorrect and would never use it (mainly Americans). Others say that this phrase can be used.
If I were you I would phone him. - subjunctive mood
If I was you I would phone him. - Simple Past

will and would in if-clauses

Conditional sentences (if-clauses, main clauses)

will in if-clauses

When the situation or action depicted in the if-clause is a result of the main clause, the will future is used in the if-clause.

He'll pay me $10 if I'll help him do the dishes. (Doing the dishes is the result of paying ten dollars.)

would in if-clauses

In polite requests would is possible in if-clauses.
It would be nice if you would help me in the kitchen.
(Are you ready to help me in the kitchen?)

English Grammar - Replacing if - Omitting if - if vs. when - in case vs. if

Replacing if

If can be replaced by words or expressions with a similar meaning.
The most common are:
as long as
assuming (that)
on condition (that)
on the assumption (that)
provided (that)
supposing (that)
unless
with the condition (that)

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Omitting if

Had I known... (instead of If I had known...)
Were you my daughter,... (instead of: If you were my daughter,...)
Should you need my advice,... (instead of: If you should need my advice,...)

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if vs. when

if and when are interchangeable when the statement of the conditional clause is a fact or a general issue (also known as zero conditonal)

If you heat ice, it melts.

When you heat ice, it melts.

if is used for something that, according to the speaker, might happen.

We can spend the afternoon on the beach if the weather is fine.

when is used for something that, according to the speaker, will happen.

I will clean up the kitchen right away when I'm back from work.

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in case vs. if

in case of can be used to shorten an if-clause as shown below:

If there is a fire, leave the room.

In case of fire, leave the room.

While if expresses a condition (1), in case is used to express a possibility (2).

(1)	I need painkillers if I'm in severe pain.
(2)	I need painkillers in case I'm in severe pain.

The expression just in case is used pretty much the same way.
I got you a pizza just in case you were hungry.
(I don't know whether you are hungry.)