1. With the adjective "happy", I can use the prepositions: with, about, that, at depending on the sentence. Is it right?
You can also use the infinitive marker- happy to see you- for- happy for him.
2. Is the expression: "on account of" formal or informal???
It's more formal than informal
3. In the sentence: "The first thing she has to do is listen to any new CDs." Why didn't the writer use "to" or "ing" before the verb "listen"? I mean, I have "verb to be" before it, so I guess I'd say: "The first thing she has to do is to listen/listening to any new CDs. Would it be possible? Why (not)???
I would say 'to listen' there.