Có những lúc chúng ta dùng hai trợ động từ. Ví dụ như:

○ He should have come.
○ They have been studying.
○ She will have finished the project.
○ You should be studying.

Trong những trường hợp như vậy, nếu dùng not thì not phải đứng sau trợ động từ thứ nhất, không đứng sau trợ động từ thứ hai.

○ He should not have come. ← đúng
○ They have not been studying. ← đúng
○ She will not have finished the project. ← đúng
○ You should not be studying. ← đúng

○ He should have not come. ← sai
○ They have been not studying. ← sai
○ She will have not finished the project. ← sai
○ You should be not studying. ← sai

Do đó, ta phải nói là would not have failed hoặc rút gọn là wouldn’t have failed. Không nói would have not failed được.