"that" and "which": losing battle, lost or imagined?

[hatwoman]

I use that and which as two different words
(This is the house that Jack built. She looked at the house, which Jack had built.)

and I hate to see them used interchangeably, but I'm beginning to wonder if I should just let it go. I just saw "which" used repeatedly in an ORT dictionary book where I would have used that. Is it just a pedantry too far? Or have I, in fact, made the whole thing up?

no pedants around tonight?

gah, I'm with you hat, but can't, sadly, tell you why.
Had a 'till/until' moment tonight.

I was more or less oblivious to this point until I began using a computer at work regularly. The now antiquated word processor program had a grammar check which I used to employ (as much for amusement as for correction). I learned quite a lot from it, in fact, including "That is almost always preferable to which in this situation". I've never forgotten.

I learned about passive voice from the same source!

I'm usually pretty fussy about grammar but I'm really confused at the that / which debate.

Old friend hatwoman, can you enlighten me on when one should use that and when which?

oh dear. now you have started my brain working..at this hour of the night too

Am also quite pedantic when it comes to grammar, and I'm debating whether "that" tends to be used in the present tense, "which" in the past (am still considering future ) but my brain-cell jury is still out.
ahhhh, too much wine

nothing to do with tenses. the point (if I have understood it correctly myself) is that "which" introduces additional information, whereas "that" indicates that what follows is crucial to the meaning of the sentence.

it's a bit difficult to get over the distinction in writing as the different meanings come across better verbally and with context - but I think this works:

I was talking about the film that I watched at my friend's house. (as opposed to the one I watched at the cinema)

I was talking about the film, which I watched at my friend's house. (here the crucial info is that you were talking about the film - the house bit could actually be lopped off, without substantially changing the meaning).

That's boggling hatwoman. I can tell your head's feeling better these days!

hat, I think the over-use of 'which' is a US English thing. Americanizers are always changing 'that' to 'which' in books I work on.

Your explanation is right I think - restrictive relative clauses (your 'film that I watched at my friend's house' - take 'that' but can also take 'which') and non-restrictive relative clauses ('the film, which I watched at my friend's house' - always take 'which').