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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' is always a conditional clause.For example 'if you don't work hard,you will fail the exam'...this automatically brings out a condition which suggests that in order to pass the examination, one must/has to work hard. On the other hand 'what if' bears a negative connotation. For example 'what if the bus does not arrive on time' means there will be a problem if the bus is late to arrive. OR say if someone asks you to stay quiet and you emphatically say that you won't you can say**'what if I don't'** rather what will happen if 'I' do not stay quiet.

Hope you will find it helpful. And if you don't, watch the film'LETTERS TO JULIET' to get the concept of 'what if' clarified.

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I don't agree with "If is always a conditional clause". It is not used for a conditional clause in "I don't know if you are right". – Rathony May 9 at 6:08

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
@CarlSmith - It is all about the name of the OP. OP just promotes his/her facebook account. – brilliant May 9 at 5:17
Still utterly confused. The answer is awful in any case. – Carl Smith May 9 at 7:08

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