Sentence structure help needed

Hello everybody
     I am wondering if it is correct to put "is" at the end of a sentence, for example.
-could you tell me what time it is?
-do you know how bad it is?
-you can never tell how red it really is!
    Also, what is the difference between, "then," and, "than"?
    Recently i hear many people ask, "did you do your homework yet?" Shouldn't they say, "have you done your homework yet?"
    Does this sentense have a correct sentense structure? -There were two people sliding on the ice. 
    Last question, are you allowed to start a sentence with a "an," for example:  An apple contains great source of energy you need for the day.
I desperately need some answers, please reply.