My daughter asked me this grammatical question, and I am not sure, though I am leaning toward it being correct:

A friend said that there are times when it's better to use "can not" because "cannot" doesn't work. What do you think? His example was, "I can not report the crime, but I should." My daughter thinks in that case it should be, "I could not report the crime, but I should" since it's conditional. I think "can not" works, but my daughter does not.

Thoughts?

To me, the two sentences have different meanings. In the cannot version, not reporting has yet taken place; in the could not, the opportunity for reporting it is past, and I would have should have at the end, rather than just plain should.

There is also a difference (if you take the gap as indicative of stress) between the original two suggestions.

I cannot report it. - It's impossible.

I CAN NOT report it. - I can choose to not report it.

Bob, you're still with us! Hope the lapse while setting up in China is brief!

To me, "could (choose to) not report the crime" is clearer. As you say, it indicates condition.

Not in China yet - not for six weeks yet. Still in Harrow.

Yes, I tend to agree with you, Bob. I think my daughter is wrong.

I agree, z, that there should be a have after should.