In this sentence, which is the correct use:
Species which/that have been regarded as varieties turn out to be new species.
(need answer fast)
That. If you use “which”, the sentence should still make sense without it.
“Species which/that have been regarded as varieties” is a defining clause, and your which/that is in the middle of it. You should use “that”.
If you’re just interjecting with an interesting-but-disposable detail, you would use “which.” Example:
“car ownership, which can cost a lot of money, is not an option for some people.”
You could scrap the entire “which” phrase and just go with:
“car ownership is not an option for some people.”
The full lesson:
Those were fast answers. Thank you very much.
I agree with those answers for American English, which is what I use. This is a convention in US English, which I believe is of fairly recent vintage.
As I understand it, it is not a convention, or less so, in other varieties of English.
You’ll find plenty of examples in any variety of English which use “which” in a restrictive clause. It’s not wrong to do so. The defining difference of restrictive vs parenthetical clauses is whether they are set off by commas. Using “that” for the former and “which” for the latter are additional signals, which can be a useful distinction.
That may be true, but if so, it’s not the case in any dialect of English I’ve ever seen and certainly not how I would ever write. In my view, “that” always denotes a restrictive clause (also called an essential or defining clause) – “I’m referring only to these things that have the described property”. Whereas “which” can be thought of as being implicitly followed by the word “incidentally” – it literally introduces incidental or supplementary information that is not essential to the meaning of the sentence.
Yes, from what I’ve read and heard, restrictive which is common in British English. That said, in colloquial American English it’s not terribly uncommon, in my experience.
That doesn’t entirely refute what I wrote – “In my view, ‘that’ always denotes a restrictive clause (also called an essential or defining clause) – ‘I’m referring only to these things that have the described property’.” But yes, it also supports what you said, this being the money quote:
In British English, however, ‘that’ and ‘which’ can both be used for restrictive clauses, allowing for a bit more flexibility. But in both the US and the UK, non-restrictive clauses are only written with ‘which’.
I honestly did not know that the Brits did this. But then, they don’t know how to spell “tire” and to them a “bonnet” is the front end of a car and a “boot” is the back end, so I wouldn’t use them as a language guide. 
Is this the beginning of an Abbot and Costello skit?
No, that is the correct use. 
On a related note, I sometimes see “what” used in places where a normal person would use “that”. Example:
The Evil Midnight Bomber What Bombs At Midnight
Is this grammatically correct and archaic, or is it just wrong?
In Britain, you can’t call dialect ‘wrong’. Calling dialect ‘wrong’ identifies you as a supporter of the traditional class system.
As it happens, I saw a joke about this on (British) television last week. A comedian with a noticeable European accent was talking about his use of ‘was’ and ‘were’: when he first came to England, all he knew of English was correct grammar, but after 20 years of living in London, he’d lost that: was he assimilated or just not speaking properly? Well, he’d been performing in regional England, and been heckled with ‘Go Back to London ya git’. Yes! Assimilated!