Does this form mustn't have plus past participle exist in English?

Answer 1

mustn't is short for 'must not'. '

must is a modal auxiliary verb and is used in connection with strong obligation and freedom to act.

'must not' is a prohibition. It is command, an instruction, a restriction on freedom to act: "Students must not copy other student's homework."

So 'mustn't' is a prohibition, concerning the present and the future.

But 'have (+ past participle)' is speaking about something that has happened in the past. It refers to a finished, completed matter.

e.g. "I have ordered tomato soup."

NOTE: It is impossible to prohibit something from happening which has already happened!

So constructions such as 'must not' + 'have done' do not make any sense!

Rather than use 'mustn't' there are several more suitable modal auxiliary verbs we can use if we want to talk about a finished past event.

• I shouldn't have ordered tomato soup!
• Oh no! I couldn't have ordered soup, because I don't like it and I'm allergic to it!
• I know I wouldn't have ordered tomato soup!

If you want to use 'mustn't' to prohibit something happening, it is used with the 'present simple' verb form.

e.g.

• I mustn't order tomato soup.
• I mustn't order tomato soup next time!
• Students mustn't jump out of the window.