How can I know it is past without đã? Just because in this sentence there is yesterday "vào ngày hôm qua", even if i don't write đã before the verb, it becomes a past?
Yes. Same with the future tense. "Ngày mai tôi (sẽ) đi".
Thank you so much
It may be coherent to accept in other sentences then if "da" is implicit
I wrote "hai tuan truoc co ay giup do chung ta", typo is not the matter, because "da" was missing, I got wrong "Hai tuần trước, cô ấy đã giúp đỡ chúng ta.
Because the action happened yesterday, it must be past tense