The declarative sentence is the most important type. You can, and often will write entire essays or reports using only declarative sentences, and you should always use them far more often than any other type. A declarative sentence simply states a fact or argument, without requiring either an answer or action from the reader. You punctuate your declarative sentences with a simple period:

Ottawa is the capital of Canada.

The distinction between deconstruction and post-modernism eludes me.

He asked which path leads back to the lodge.

Note that the last example contains an indirect question, "which path leads back to the lodge." An indirect question does not make a sentence into an interrogative sentence -- only a direct question can do that.