I've been told that in complex sentences one must use "that" rather than "which" to refer the whole previous subsentence or a word in it, that is not right near to the preposition.
For example I couldn't use "which" in the sentence above:
one must use "that" to refer a word in an subsentence, that is not right near to the preposition.
Or I can't write:
I love fish and vegetables, which (fish and vegetables) is the most common food in Japan.
Is that a correct statement? When "which" can be used as a preposition in complex sentences?
Best Answer
First, let me just correct your second sentence:
Fish and vegetables are plural, so the rest of the sentence has to agree. Now, the concept to understand is that "which" is non-restrictive, whereas "that" is restrictive. Have a look at these:
The first one says two things: I want to go to an Ivy League school. Ivy League schools are among the highest rated in the world. The first sentence doesn't restrict the first one; it provides additional related information.
The second one says only one thing: I want to go to an Ivy League school, but only an Ivy League school that is among the highest rated in the world (by implication, not one of the other Ivy League schools that is not among the highest rated in the world). This sentence is restrictive; the "that" clause restricts or qualifies the main clause.
Here is an article that covers the basics of the subject and gives more examples.