Use a period at the end of a command.
- Hand in the poster essays no later than noon on Friday.
- In case of tremors, leave the building immediately.
Use a period at the end of an indirect question.
- The teacher asked why Maria had left out the easy exercises.
- My father used to wonder why Egbert’s ears were so big.
Use a period with abbreviations:
- Dr. Espinoza arrived from Washington, D.C., at 6 p.m.
Notice that when the period ending the abbreviation comes at the end of a sentence, it will also suffice to end the sentence. On the other hand, when an abbreviation ends a question or exclamation, it is appropriate to add a question mark or exclamation mark after the abbreviation-ending period:
- Did you enjoy living in Washington, D.C.?
Occasionally, a statement will end with a question. When that happens, it is appropriate to end the sentence with a question mark.
- We can get to Boston quicker, can’t we, if we take the interstate?
- His question was, can we end this statement with a question mark?
- She ended her remarks with a resounding why not?