a little adjustment to rechels last answer. correct didn't is past tense and wont is future tense but isn't is often used for present and future

those three are sometimes used interchangeably.. these are negative contractions.. isn't = is not didn't = did not won't = will not but somehow "isn't" is more associated with the use of present tense and the word "is".. "didn't" with past tense and "won't" with future.. hope this'll help you a little..^_^

All are negative contractions. "isn't" 1. There isn't any water in the jug. 2. She isn't in our team. 3. A: Is he your English teacher? B: No, he isn't. "didn't" 1. I didn't go to the library last night. 2. A: Did Sarah get up early this morning? B: No, she didn't. 3. We didn't want to go to the park with John. "won't" 1. I won't see the doctor on Saturday. 2. Mr. Lee won't give the test results tomorrow. 3. A: Will you be at the party tonight? B: Sorry, I won't. I'm busy.

You can use it any time. It is perfectly correct, formal enough. The only time you might prefer to use the whole words is if you want to emphasize one of the words by itself. You ARE going to do your chores! YOU are going to do your chores! (not your brother because you paid him) But otherwise there's no reason at all to not use.... let me start again. There's every reason to use the contraction forms. They sound crisp and correct, and although they are 'shortened', they are not sloppy nor shortcuts. You might find uncontracted forms a lot more in old literature, which reflected maybe how people spoke. But that's an artifact of english literature, not a style to emulate (unless you want to sound like Jane Austen) I am perfectly certain that I shall indeed have the pleasure of your company. etc etc We don't actually talk that way any more. You have to wear these giant petticoats and bustles and hats in order to talk that way.