Is it true that "for" indicates the whole duration of a verb? What about "he was ill for 3 days"?

Is it true that for indicates the whole duration of a verb?

What about the following?

He was ill for three days when I turned back from my journey.

Solution 1:

It's not the word for that you need to correct here, it's the verb tense.

What you want to say is that you turned back after the third day of his illness, and when he had not yet recovered. In this case, you should say:

He had been ill for three days when I turned back from my journey.

Grammatically, this doesn't necessarily mean that he had not yet recovered, but the context implies it.

The sentence

He was ill for three days when I turned back from my journey,

is very unlikely to be spoken by a native English speaker. What it seems to mean is that he was ill for three days during or after the time you turned back. A similar sentence that a native English speaker might use is:

He was ill for three days when he ate the clams at the Dirty Diner.