Can I use an "if" clause without "then"?

It is not necessary to use then to introduce the consequence of the if clause:

If you build it, they will come.

If it ain't broke, don't fix it.

If I give you five dollars a week, you'll have over $250 by the end of the year.

All those are grammatically correct and clear (even the one with ain't, which I threw in for a grin.).

You can easily answer this backwards.

Consider

I will go, if you go.

I can simply flip it and say

If you go, I will go

I don't see myself obligated to write "then" in the second sentence!
If that is true, then I should have "then" in the first sentence, too! Your teacher is probably trying to come up with a reason why he took points off.

You. He/she is just being pedantic.

• http://www.fortunecity.com/bally/durrus/153/gramch28.html
• http://en.wikipedia.org/wiki/Conditional_sentence
• http://www.eslhq.com/forums/esl-forums/english-questions/if-then-clause-10627/