Dong, Musti/Harus, Yang lalu

Dong
Dong is used to emphasise an idea expressed. For example:

Eg: Anda mau bermain bola basket? Ya, mau dong!

In this case, "Ya, mau dong!" means, "Yes, I really want to!"

The difference between Musti and Harus
Musti is used informally. It is only used verbally. Harus is formal, and can be used verbally or in writing.

Yang lalu
Yang lalu is used when talking about something in the past. For example:

Dua tahun yang lalu, umur saya sembilan puluh tahun. (2 years ago, I was 19 yrs old).

Pada pesta yang lalu, saya ketemu Sue. (At the last party, I met Sue).