He had done
He had been doing
He had done
He would do
He would be doing
He might do
He might be doing
He could do
He could have done
He must do/had to do
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Lùi 1 thì của động từ thường: QKĐ -> QKHT; HTHTTD -> QKHTTD;...
Lùi 1 thì của modal verbs: can -> could, will -> would, may -> might, have to -> had to,...
Đặc biệt: must -> must. Nhưng ta thường đổi "must" thành "have to" rồi lùi thì thành "had to".
