Here are the correct uses of "since" and "for" based on the sentences provided:

1) **for** - My sister has been a student **for** two years.
2) **since** - We’ve lived here **since** 1999.
3) **since** - I haven’t seen Linh **since** my birthday party.
4) **for** - She’s been away **for** five days.
5) **for** - We’ve already waited **for** a year.
6) **for** - They have only been there **for** a few minutes.
7) **for** - We haven’t had any rain **for** a very long time.
8) **since** - That building has been there **since** the 19th century.
9) **for** - I have known them **for** many years.
10) *The sentence can be retained as is, but doesn’t require "since" or "for."*

For the rewrite, it can remain: "It’s three years since I last saw Nam."

If you need further clarification, feel free to ask!