Saying it already happened: đã

Vietnamese verbs never change for time. Tôi ăn can mean "I eat" or "I ate"; the situation, or a time word, tells you which. Đã before the verb says clearly that it happened, or has happened: Tôi đã đặt bàn. I've booked a table.

You don't need đã in every past sentence. When a time word already says when, people usually leave it out: Hôm qua tôi đi Vũng Tàu. Use đã when nothing else shows it's past, or to stress that it really did happen.

In everyday speech, especially in the South, rồi at the end is the more common way to say "already": Tôi ăn rồi. The two often go together: Em đã gửi email rồi. Before an adjective, đã means something has become so: Em đã khỏe hơn rồi. I'm feeling better now.

Đã can't stand alone as an answer: for "yes, I have", use rồi. For "not yet", use chưa. For "didn't", không before the verb is usually enough, with no đã: Hôm qua tôi không đi làm. For something that has only just happened, use vừa or mới: Tôi mới về. Đang and sẽ do the same job as đã for now and for later. At the very end of a sentence, đã means something else, "first": Chờ chút đã. Wait a moment first.