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Must have, might have, and cannot have for conclusions about the past.
Why you specifically
Trong tiếng Anh, khi đoán về một sự việc đã xảy ra trong quá khứ, ta thường dùng cấu trúc: modal + have + past participle (động từ ở dạng quá khứ phân từ). Ví dụ: must have gone, might have lost, cannot have finished. Must have dùng khi người nói gần như chắc chắn về một kết luận trong quá khứ, dựa vào dấu hiệu hoặc bằng chứng: She must have missed the train. = Chắc là cô ấy đã lỡ tàu. Might have dùng khi chỉ là khả năng, chưa chắc chắn: She might have missed the train. = Có lẽ cô ấy đã lỡ tàu. Cannot have dùng khi kết luận rằng việc đó không thể xảy ra hoặc rất khó xảy ra: She cannot have missed the train because she is already here. = Cô ấy không thể đã lỡ tàu vì cô ấy đã ở đây rồi. Điểm rất quan trọng là sau have phải dùng past participle, không dùng động từ nguyên mẫu. Vì vậy không nói must miss, might lose, cannot finish để diễn tả phán đoán về quá khứ. Cũng không bỏ have trong các cấu trúc này. Trong IELTS, lỗi phổ biến của người học Việt là dịch trực tiếp từ tiếng Việt nên quên rằng tiếng Anh phải có have + V3 để nói về suy đoán quá khứ.
The rule
Modals of deduction in the past are used to guess or conclude what happened earlier. The structure is modal + have + past participle: must have for a strong conclusion, might have for a possible conclusion, and cannot have for a conclusion that something was impossible or very unlikely. After have, English needs the past participle, not the base verb. So you say must have gone, might have lost, and cannot have finished. These forms are about a finished past event, not the present. Use must have when the evidence is strong: She must have missed the train. Use might have when you are unsure: She might have missed the train. Use cannot have when the conclusion is negative and strong: She cannot have missed the train because she is already here. A common Vietnamese error is to use the base verb after must/might/cannot, or to translate directly as “must” without the extra have. In English, the past meaning must be shown with have + past participle.
The trap
She must miss the train because she is late.
She must have missed the train because she is late.
Use must have + past participle to make a strong deduction about a finished past event.
He might forget his keys, so he cannot get into the house.
He might have forgotten his keys, so he cannot get into the house.
Use might have + past participle for a possible past reason, and cannot have + past participle for a past conclusion that is impossible or very unlikely.
They cannot leave yet because their car is still outside.
They cannot have left yet because their car is still outside.
Cannot have + past participle is used to say a past event was impossible or highly unlikely. The base form cannot express this past deduction.
I think she must have tired after the long flight.
I think she must have been tired after the long flight.
After must have, use a past participle. Be careful not to use an adjective or base verb after must have.
Prove it
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