Use a period at the end of a complete sentence that is a statement.
I know that you would never break my trust intentionally.
If the last word in the sentence ends in a period, do not follow it with another period.
I know that M.D. She is my sister-in-law.
Please shop, cook, etc. I will do the laundry.
Use the period after an indirect question.
He asked where his suitcase was.