What is the correct tense to follow "I've known"?

Question

I'm unsure about what tense should be used in a clause that follows 'I've known'. I've seen it followed by the present perfect, the simple present, and the simple past. I don't know which of the following is best or if all could be used depending on the context.

'I was recently diagnosed with ADHD, but I've known that I've had it for years.'

'I was recently diagnosed with ADHD, but I've known that I have it for years.'

'I was recently diagnosed with ADHD, but I've known that I had it for years.'

'I've known that she has been pregnant for a while.'

'I've known that she is pregnant for a while.'

'I've known that she was pregnant for a while.'

I think the top of each three might be best, but what has me confused is that if I change the word order, using the simple present suddenly sounds a lot better:

'I was recently diagnosed with ADHD, but I've known for years that I have it.'

'I've known for a while that she is pregnant.'

Why is that? Why does the change in word order make it more acceptable?

Answer 1

Let's first deal with the position of "for years" and "for a while". When it appears at the end of the sentence, it's not clear whether that phrase modifies the verb closest to it or "have known". Sometimes context will make the reference clear, but in this case either meaning is possible. This "attachment ambiguity" (or misplaced modifier, or whatever you want to call it) can affect the selection of verb tense and aspect. (For example, in the pregnancy sentence try to change "for a while" to "last year".) I think that you want the phrase to modify "have known", so I'll deal with these sentences:

I was recently diagnosed with ADHD, but I've known for years that I have it.
I've known for a while that she is pregnant.

In both cases, you could use either the simple present (as above) or the present perfect:

I was recently diagnosed with ADHD, but I've known for years that I've had it.
I've known for a while that she's been pregnant.

It is also common to mix the simple past with the present perfect:

I was recently diagnosed with ADHD, but I've known for years that I had it.
I've known for a while that she was pregnant.

I agree that the first of these options (i.e., using the simple present tense) sounds best. In that case, why does the present perfect sound better when the modifier is moved to the end of the sentence:

I was recently diagnosed with ADHD, but I've known that I've had it for years.
I've known that she has been pregnant for a while.

I suspect that because the two verb phrases ("have known"/"have had" and "have known"/"has been") appear closer to each other, we expect the tenses to be consistent; different tenses seem to clash in such close proximity. Nevertheless, any of the three options (simple present, present perfect, simple past) is possible.