I sometimes cannot fall asleep.
I cannot sometimes fall asleep.
“I cannot sometimes fall asleep” is the correct positioning of this sentence. Sometimes is an adverb of frequency. Adverbs of frequency include often, rarely, never, always, usually,etc.
The rule for the position of adverbs of frequency is they are usually placed before the main verb after a modal verb such as would, should, can, may, etc.
Let’s take a look at some examples:
- They always go to the cinema on Friday nights.
- I never travel by bus.
- The teachers occasionally bring sweets to class.
- He should never hang out with those people again, they are dangerous.
- We can always tell when they are lying.
- I would never quit my job, I love it here!
There is an exception to this rule, if the verb to be is used the adjective must be placed AFTER the verb. For example:
- I am always on time.
- We are never absent from class.
- They were sometimes sent to the principles office.
Some adverbs of frequency can be placed at the beginning or at the end of a sentence to emphasise something.
- Sometimes I can’t concentrate.
- I can’t concentrate sometimes.
- Occasionally they fight.
- They fight occasionally.
Therefore, you could say,
- Sometimes I cannot sleep.
- I cannot sleep sometimes.
This will put emphasise on the fact that you can’t sleep at times.