In English, "which" is expressed using two words - 'which' and 'that'. How to choose the right one? Let's not rush and first deal with two types of subordinate (i.e. dependent) sentences, before which - sorry for the tautology - our "which" is placed.
The first type of subordinate clauses - restrictive or clarifying
This type of subordinate clause introduces essential information into the sentence itself, without which it loses its meaning and main meaning. For clarity, we give a couple of examples.
Chairs that don't have cushions are uncomfortable to sit on.
Card games that involve betting money should not be played in school.
To our knowledge, it is the only body in the solar system that currently sustains life…
The highlighted dependent clauses are restrictive. How to recognize them? Yes, it's very simple - try to omit them, and the whole sentence will immediately lose common sense, become illogical and meaningless.
The second kind of clauses are descriptive or non-restrictive.
Such sentences are "served" either in brackets or in commas. If such a subordinate clause is omitted, then the whole sentence will not lose its meaning.
Chairs, which are found in many places of work , are often uncomfortable to sit on.
I sat on an uncomfortable chair, which was in my office .
It can be seen from the examples that 'that' is used with restrictive clauses, and 'which' is used with non-restrictive clauses.
Why is it important to understand the difference between that and which?
Replacing that with which and vice versa can completely change the meaning of a sentence. Look at two seemingly identical examples.
My car that is blue goes very fast.
My car, which is blue, goes very fast.
The first sentence assumes you have more than one car. If we omit this clause from that, we get just "my car is going very fast." The meaning of the sentence has completely changed: the reader does not know which of my cars drives fast.
At the same time, the clause with which simply informs the reader that my car is blue. Therefore, if we “pull out” the subordinate clause from it, then the meaning of the whole sentence will not change significantly.
How and when to use that and which
Today it is quite common to use which in both kinds of clauses, especially in colloquial speech.
Who ate the cake that I bought this morning?
Who ate the cake which I bought this morning?
However, even in colloquial speech it is considered a gross mistake to use that in non-restrictive clauses. For example, these two sentences are incorrectly constructed.
This computer, that I have never liked, is very slow.
The blue desk, that my father gave me.
Despite such indulgences, you should not abuse it - it is better to adhere to the basic rule described at the beginning. It is suitable for both spoken and written language.