To express a hypothesis

The past subjunctive is used after the conjunction if in a contrary-to-fact protasis. For example:
If I were a millionaire, I would buy a sports car.
If he had a car with him, he could drive us there.
If I were a rich man...

In the same vein, the past subjunctive is used following the conjunctions as if and as though to express a contrary-to-fact situation that reality is supposed to resemble:
She looked as though she were going to kill him, but after glaring for a bit, she just stormed off.
He tried to explain it — as if he knew anything about the subject!

As seen in the second of these examples, these constructions are often used derisively.

Note that the past subjunctive is sometimes used in expressing situations that are not necessarily contrary to fact:
? I'm torn; if I were to go with choice A, I'd be better off in the short term, but if I were to go with choice B, I might be better off in the long term.
? Bring an umbrella; looks as if it were going to rain soon.

However, such uses are not universal, and are often regarded as ungrammatical.[