Compare these two sentences. Is there a difference?
The TV which I ordered three weeks ago has just arrived.
The TV that I ordered three weeks ago has just arrived.
Well done if you said 'no difference'. Either which or that is possible here. In fact, both words could be removed and the sentence would still be OK:
The TV I ordered three weeks ago has just arrived.
Now try these two sentences.
The TV, which I ordered three weeks ago, has just arrived.
The TV, that I ordered three weeks ago, has just arrived.
Well done if you noticed something not quite right about the second sentence. Here only 'which' is correct. And it cannot be omitted.
In the first example, the 'that/which' clause is essential to the meaning of the sentence. The clause is attached to the TV. It is a defining clause.
In the second example, the clause becomes a secondary or added detail. The clause is surrounded by commas to show that it is not essential to the TV. It is a non-defining clause.
So the commas make all the difference.
For defining clauses (usually without commas) use which or that.
For non-defining clauses (usually with commas) use which.
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