Unreal Present and Unreal Past

· If I were smarter, I would have graduated from Stanford.
( But I am not smarter and therefore I didn't graduate from Stanford.)

· If Mary weren't a snob, she wouldn't have had so many parties this year.
(But Mary is a snob and therefore she had so many parties this year.)

6. ‘If...not’ and ‘unless’

If not and unless are sometimes interchangeable but there are
occasions when it is impossible to use one in place of the other

When ‘if...not’ and ‘unless’ are interchangeable

Both if not and unless can be used in negative Type 1 conditionals without a noticeable change of meaning:

If you don't change your mind I won’t be able to help you.

Unless youchange your mind I won’t be able to help you.

However unless is stronger than If not and is sometimes preferable:

Unless the management improve their offer there’ll be a strike.

When we cannot use ‘unless’ in place of ‘if...not’

Unless always means ‘except on the condition that’
so we cannot normally use it to refer to unreal situations:

She’d be better company if she didn't complainso much.

Unless is not used in most questions and when we talk about emotions:

If you don’t pass the test, what will you do?

I’ll be amazed if Christie doesn’t win.

When we cannot use ‘if...not’ in place of ‘unless’

We often use unless in past references to introduce an afterthought
The unless - clause follows the main clause and is usually separated by
a dash rather than a comma:

I couldn t have got to the meeting on time — unlessof course I had caught an earlier train.

This means the speaker didn’t get to the meeting. He could only have
done so by catching an earlier train. If we use if not in place of
unless in the above sentence we get

I couldn’t have got there if I hadn'tcaught an earlier train.

The sentence now conveys the exact opposite meaning: the speaker
didget to the meeting because he didcatch an earlier train [6; 289].

‘If’ and ‘unless’ clauses in short answers

Note how if-clauses and unless-clauses can occur in short answers:
-Will you help us with all this re-decorating?
-Yes, if I can. No, not unless you pay me.