A difference between the conjunctions "why" and "when"

Question

I think both the following sentences are correct:

A. The reason that I asked the question is that I am interested in the subject.

B. The reason why I asked the question is that I am interested in the subject.

But only statement A among the following two sentences is correct:

A. The last time that I saw him, he was tired.

B. The last time when I saw him, he was tired.

Am I right with the above evaluations? If so, why are "when" and "why" subject to different grammatical rules?

One explanation I can think of myself is the following. The second pair includes the adjective "last" and that is the main reason for the difference. If we add a similar adjective to the word "reason" in the first sentence, then only the case with "that" will be acceptable even in the first pair. — H D, Commented Mar 19, 2022 at 14:20
I think you're onto something, with that comment, HD. The time when I saw him is fine. — Colin Fine, Commented Mar 19, 2022 at 14:52
@ColinFine Thank you for this! I appreciate it! I wonder how natural the following sentences sound to you: "The most important reason why I asked the question is blah blah" or "The major reason why I asked the question is blah blah". I am trying to test the other side of my hypothesis. That is, the presence of an adjective before the word "reason" makes the conjunction "why" unacceptable. — H D, Commented Mar 19, 2022 at 18:43
No, they sound OK to me. (Mind you, my father objected to the reason why in any context: he thought it was redundant). — Colin Fine, Commented Mar 19, 2022 at 18:52
@MarcInManhattan Thank you for your help! I really appreciate it. As you alluded to, Google books suggests the phrase "the last time when" does occur in the real world. But, it seems like the examples typically fall under one of the following two categories: (i) Relatively old texts (Late 1800s, early 1900s) (ii) Technical texts. But anyway, they do occur. — H D, Commented Mar 23, 2022 at 5:37

swmcdonnell · Accepted Answer · 2022-03-27 19:48:23Z

1

At least in spoken American English, all four examples are permissible. In the last example, which you think is a problem, you could also say:

When I saw him the last time, he was tired.

and that would be fine, too.

answered Mar 27, 2022 at 19:48

swmcdonnell

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