Gone, lost: verb + mất

After a verb, mất adds a sense of loss: something is gone, or it happened when you didn't want it to. Em quên mất! I completely forgot! Ai ăn mất cái bánh của em rồi? Who's eaten my cake? You know mất as "to lose" and, with time, as "takes"; here it follows another verb.

Most often rồi comes after it, for something already gone: Xe buýt chạy mất rồi. The bus has left. Cái áo đó bán mất rồi chị. That shirt's already been sold. Without rồi, it can warn that a loss is coming if nobody hurries: Nhanh lên, trễ mất! Hurry, we'll be late! (in Hà Nội, muộn mất).

Quên mất is the one you'll use most. It's stronger and more apologetic than plain quên: Chết, em quên mất! Oh no, I totally forgot!

For losing something yourself, you can say mất on its own, Em mất điện thoại rồi, or làm mất, which makes clear it was your doing, even by accident: Em làm mất chìa khóa rồi. I've lost my key.