They're very close, so I know this is hair-splitting. But there is a difference, which Chicago explains well.
First, understand that I'm only talking about American English, not British English. Since you're probably using British English, this is purely academic, right?
Here are two versions of one of your sentences. Both use which, since you prefer that word, but only one has a comma:
1. Without a comma: Yesterday evening I logged on to a website which I found very useful to us English majors and I would like to recommend to you. [In American, for reasons you read above, it would use that.
2. With a comma: Yesterday evening I logged on to a website, which I found very useful to us English majors and I would like to recommend to you.
There is a difference.
In the first, which clearly refers to the website you logged onto. And that is what you meant. That particular website was useful. The second, with a comma, might refer to the act of logging in and is not about one particular website. That is not what you meant. Right?
In American English, we would only have used which only if there had been a comma. Without the comma (as you correctly have it), it would require that.
So, because I did not know then that which is correct in British English, I changed it to that. That's why I say in my profile that I'm an American.
Sorry for the error!
This post of yours seems to contradict with what you said in post # 23.
In this post you say, "In the first,
which clearly refers to the website you logged onto. " But in post # 23 you said, " If you say
that (...a website that I found useful
), it's clear that the website itself was useful" .
How come both which and that clearly refer to the website I logged onto? ! Is it that I misunderstand your meaning here?
In post # 23, you also said, “If you say
which, it's not clear whether a particular website was useful, logging into that website was useful, for the act of logging in, regardless of the website, was useful.”
In contrast, in the current post, you say, “
which clearly refers to the website you logged onto. ”
You also say, “The second(Yesterday evening I logged on to a website, which I found very useful to us English majors and I would like to recommend to you.), with a comma, might refer to the act of logging in and is not about one particular website.” Does this understanding really happen? I assume that it rarely takes place, for
our common sense works forcibly to restrict what this which actually refers to. I would like to take an example sentence from Collins COBUILD English Usage again for proof. The sentence goes, “He was a man of considerable inherited wealth, which he ultimately spent on his experiments.” Our common sense tells us that it couldn’t be [his] being a man of considerable inherited wealth that was spent on his experiments and that it is only possible and sensible to understand that it was his considerable inherited wealth that was spent on his experiments. And I firmly believe that our common sense works the same way with another example sentence on the same page (p.580)
I know that you meant to say that according to American English usage, all the which’s should be changed to that and the sentences in my OP do not need to be changed to ones containing non-restrictive relative clauses.
By the way, in post # 23, you repeatedly reminded me that a comma should be used in a compound sentence before the conjunction, but
you have forgotten to add it in your two sentences:
1. Without a comma: Yesterday evening I logged on to a website which I found very useful to us English majors
[,]and I would like to recommend to you. [In American, for reasons you read above, it would use
that.
2. With a comma: Yesterday evening I logged on to a website, which I found very useful to us English majors
[,]and I would like to recommend to you.
Anyway, I do not mean to belittle your contribution to a global understanding of this language issue here. In fact I highly appreciate your help, which I believe surely leads me to achieve a thorough understanding of this issue. If this post of mine makes you feel offended, I will apologize for it. Thank you very much.