No đã or sẽ needed when a time word says when

Vietnamese verbs never change for time, and you don't have to add đã or sẽ either. When a time word already says when, people usually leave them out. Hôm qua em ăn phở is "I had phở yesterday"; Mai em ăn phở is "I'm having phở tomorrow". Only the time word changes.

The same goes for states and habits: Bây giờ tôi ở Sài Gòn. I live in Saigon now. Năm ngoái em học ở Úc. Last year I studied in Australia.

Even without a time word, the situation often says enough. Telling a friend about your weekend, Tôi đi biển already means "I went to the beach".

Adding đã or sẽ anyway isn't wrong. Đã stresses that something really did happen, and sẽ makes the future explicit and can sound more definite: Mai em sẽ gọi. I will call tomorrow. Đang is different: people often keep it even with a time word, when they mean "right in the middle of it": Lúc đó tôi đang ngủ. I was asleep at that moment. What English speakers have to unlearn is the feeling that every past sentence needs đã: it doesn't. To say something is already done, people usually add rồi at the end: Em ăn rồi. I've already eaten.