I may have seen on here, once or twice, unsolicited grammar lessons. 
So I thought it would be naice to have a safe place for grammar help.
If you don't know your arse from your aspostrophe, or your stushe from your subordinate clause, please feel free to ask on here with impunity. 
Also, if you can offer guildance on here, please feel free to pop along and gently steer.
I'll start with two questions and one lesson.
If I would like to say "Oh, yes, I went to their house, and had a super time" but wanted to miss out the word house, does their become "their's" or "theirs"?
I'm never sure which is correct in this instance - when at MacDonald's the pub and they ask "What drink do you want?", I'm always sure it should be "Which drink do you want?", but I can't articulate why. Please help. 
And my mini lesson. The word "it's" is only ever used to denote missing letters, and not to indicate belonging/possession. So "the cat licked it's arse" is bang wrong.
We probably do with Bof here for her semi-colon masterclass. 
Please come and join me for Grammar Fun
but don't mention the typos.