If I reply to someones tweet that contains a hash tag do I have to write the hash tag at the end of my tweet?

[Windowphobe]

You don't have to include the hashtag unless you want your reply to be included in searches for that topic. (Usually, though, you do.) I prefer to stick the hashtag at the very end of my comment, though it really doesn't matter where it is as long as the @userid of the person you're replying to comes first.