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My English teacher asked me what's the difference between what if and if last week.
I can't search anything about that. What only I know is what if is a question sentence. And if is a general(?) sentence.

Is it related with class word (a part of speech)?

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up vote 5 down vote accepted

In both cases you have a conditional sentence. What if means What will/would happen if... You are right when you say it is used in a question. If on the other hand is used to introduce a conditional clause, not necessarily a question. Here are two examples:

If I told you the truth, what would you do?

You can also have a positive or negative sentence with if of course, as you suggested in your question: If I told you the truth, you'd be hurt. OR If I didn't tell you the truth, I would feel awful.

What if I told you the truth? (This implies the question of what you would do or what would happen.)

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If:Conditional Clause: If you put your finger into hot water, you burnt. If you eat much sugar,your teeth are decay. What if: Question: "What if?" Answer: Last night, I had a dream. There was litter all over Singapore. Giant rats and cockroaches are everywhere. One giant cockroach came towards me. It turned and ran as fast as I could. The cockroach was about to catch me when I woke up.

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Sorry, but that doesn't make any sense to me at all. What has the horror story got to do with the question? – Carl Smith Nov 1 '14 at 13:02

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