if

In We have hundreds, if not thousands, of items in stock, the if not fairly plainly means "or even." In It's a clever idea, if not a practical one, it fairly plainly means "although not a practical one." But in He's good-looking, if not really handsome, it is unclear which of those meanings is intended - at least out of context. Often it is clear what if not means only because the context shows what the phrase must mean. When you think it may be unclear, choose another wording.

In an if-clause expressing a condition contrary to fact, you must use the subjunctive mood of the verb in that clause (If I were [not "was"] you), and you must use the modal auxiliary verb would (or, less commonly, should) in the main clause: If I were you, I would not try that at home. When the if-clause expresses a condition not contrary to fact, you must use the indicative mood of the verb in that clause: If Jon was still on the road during the blizzard, he probably tried to stop at a motel for the night. Notice that the mood and tenses of the main-clause verbs are dependent on the verbs in the if-clauses. The sense is that we know that Jon has been on the road. We are not sure, however, whether he is still there. Hence, we can use the indicative, not the subjunctive.

Substituting would have for had in an if clause (one stating a condition contrary to fact) is a grammatical error. Do not write: If they would have done it properly to begin with, these problems would not exist. Write instead: If they had done it properly... or, more formally, Had they done it properly to begin with, these problems would not exist. Avoid the incorrect form they'd + have, as in If they'd have done it properly.. Here they'd is a contraction for they had. Write instead If they'd done it properly... or If they had done it properly.