Question

He can not help doing it . ( use 'must' )
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Answer 1

Answer:

can’t help doing something” is an interesting English expression. The form is:

can’t help + gerund (spoken form)

cannot help + gerund (more formal written form)

Here is an example of its use:

Sarah often shouts at me but I can’t help liking her

Explanation:

:D

Answer 2

Explanation:

He must not help doing it.