Genitive: use the apostrophe between the noun and the s, unless the noun is plural, in which case you use use the apostrophe after the s. For nouns where the plural doesn't end in s, i.e. children, you put the apostrophe between the plural noun and the s. Note: People get confused about the word "its" because the genitive form (meaning "belonging to it") has an s but no apostrophe. If you see "it's" with the apostrophe, that form is a contraction meaning "it is."
As for appositives, your example really isn't one -- since it starts with "who," it's a relative clause. An example of an appositive would be: "Susie, a beautiful girl, came to my party." In both your example and mine, though, you would set the clause/appostive apart by commas because you don't need the information enclosed in the commas for the sentence to retain its meaning.
> since it starts with "who"
OK :). At least my commata were right. Can you sum up on a few lines where a comma is due? No prob if some exotic thing is missing. Just write what you think a foreigner must at least get right.
Can you also say something to my "an hour"/"a hour" question, please?