We all play different roles in various aspects of our lives. Any one of us can go through the day wearing different hats (writer, editor, spouse, cook, home maintenance, some of us are parents, etc.). And like people, words play different roles depending on their context in a sentence. While this seems obvious, it can cause confusion and misuse of terms, which brings about ambiguity or potential misreading.

For instance, the terms that and which are prone to misuse when used as relative pronouns introducing adjective clauses. Keeping a few simple rules in mind will help eliminate confusion so you can use these words correctly. Take a look at the following examples:

The bedrooms that we painted during the summer look cheerful and bright.

The bedrooms, which we painted during the summer, look cheerful and bright.

Both of these sentences describe the bedrooms, but what the first sentence tells you is completely different from what you learn from the second one. In the first sentence, you see multiple bedrooms but only the recently painted ones appear cheerful and bright, but the second one says that all the bedrooms look cheerful and bright and mentions that they've been painted.

The Restrictive Clause
The adjective clause in the first example sentence above (that we painted during the summer) is called a restrictive clause because it limits the meaning of the nouns it modifies. The restrictive clause introduces information that is essential to understanding the meaning of the sentence, and that is the relative pronoun normally used to introduce this clause.

The Nonrestrictive Clause
The adjective clause in the second example sentence (which we painted during the summer) is called a nonrestrictive clause because it modifies the noun but does not necessarily limit its meaning. All of the bedrooms look cheerful and were painted. The nonrestrictive clause introduces information that could be helpful but is not essential to the understand the sentence, and which is the relative pronoun usually used to introduce the nonrestrictive clause. And as you can see from the example, the nonrestrictive clause should be separated from the rest of the sentence with commas.

Exception to the Rule
Like a number of grammatical rules in English as well as other languages, this one has an exception, but it should only be used when a sentence has more than one dependent clause or when that has been used in another role. Consider the following example:

That concept, which has been discussed thoroughly, no longer needs to be addressed.

If this, that, these, or those has already been used as an adjective or if that has introduced the first clause, use which to open the next one, whether the information is essential or not.