Which is the correct preposition in each of the sentences?

I would use "in" for all of those sentences, although "at" would be acceptable in the first sentence.

Now learn the dative case. Suppose I say "Give me your book." The word 'me' is in the dative case because what I mean is "Give TO me your book." The dative case exists any time you have in, to, on, for, by, from, of, and so on.

In some languages they don't have anything to indicate subtle shades of meaning. For instance Spanish makes no distinction between the knife "on" the table and the knife "in" the table.

In English we have certain preferred words, but only because we happen to prefer them. We say a person is good "in" intellectual activities, but good "at" physical activities, and we don't insist on following either rule.

Thanks, Setanta/

Thanks, Jewels Vern.