Interesting topic name, I see no "usually" or "always". Anyways, I'll respond with a general response. Usually is like 90%, whereas always is 100%.
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WwwdotcomYour question answers my view of this. You used "WHICH", you did not use "THAT". Why? Because you don't know. So, let's look at each question and see how frequently they are used instead of RIGHT vs. WRONG.
GROUP A
1. I don't use this, but I have seen it. So, I would mark it as 25%
2. I would use this, and I see it used quite often, 75%
WwwdotcomI don't see anything wrong with using that or which. However, I think "which" has more of a question aspect to the sentence. Using "that" is more established between the listener and speaker. For example, I may say to a friend, "Hey, you know that new mp3 player that/which I got last week? Well, it's now on sale for half the price." Either is ok, but I