[Grammar] had difficulty trying/tried

I think the answer to the following is “trying”.

It was already 2am. Chris had difficulty trying to keep awake.

Why the answer can’t be ‘tried’

Thank you for teaching me! :-D

It is because 'in' has been omitted from 'Chris had difficulty in trying to keep awake', where 'tried' cannot be used.

You cannot have two verbs one after another after a subject - "had" and "tried".

I think that also applies to the present tense, as in 'I have difficulty (in) trying to ...'

I don't think it's the "trying" he's having difficulty with. He's simply "having difficulty staying awake".

Matthew Wai said:

I think that also applies to the present tense, as in 'I have difficulty (in) trying to ...'

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"Trying" is not a verb in the above but a noun(gerund).

emsr2d2 said:

He's simply "having difficulty staying awake".

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Agree.
I also would say that as a native I would never say "trying to keep awake" but "trying to stay awake" instead.

tedmc said:

"Trying" is not a verb in the above but a noun(gerund).

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It is intended to show that 'trying' should be used instead of 'try', a verb in the present tense.
'Trying' is a participle instead of a gerund if 'in' is omitted.