Perhaps Susan knows the address. (may)
Susan may know the address. (Modal verb "may" diễn tả khả năng có thể xảy ra)
The report must be on my desk tomorrow. (has)
The report has to be on my desk tomorrow. ("Have to" diễn tả sự bắt buộc, tương đương "must" trong ngữ cảnh này)
It was not necessary for Nancy to clean the flat. (didn’t)
Nancy didn’t have to clean the flat. ("Didn't have to" diễn tả việc không cần thiết phải làm gì trong quá khứ)
The best thing for you to do is to sit down. (better)
You had better sit down. ("Had better" diễn tả lời khuyên mạnh mẽ, nên làm gì để tránh hậu quả xấu)
I think you should give up smoking immediately. (had)
I think you had better give up smoking immediately. (Tương tự câu 4, "had better" diễn tả lời khuyên mạnh mẽ)
It’s necessary for me to bring my passport. (have)
I have to bring my passport. ("Have to" diễn tả sự cần thiết, bắt buộc)
I’m sure you are very tired after working so hard. (must)
You must be very tired after working so hard. ("Must be" diễn tả sự suy luận logic dựa trên bằng chứng)