Adjective clauses are clauses that describe a noun.

For example:

• I have found a shirt that costs less than twenty dollars.

The phrase, "...that costs less than twenty dollars" tells us information about the shirt. That is the function of an adjective, whether it is one word or a phrase or a clause. When the noun that the adjective clause describes is definite (a particular one that we have identified clearly in our mind), then the verb is in the indicative. If the noun is not definite (we don't have a particular one in mind), then the verb in the adjective clause is in the subjunctive.

Let's look at the first example:

• "I have found a shirt that costs less than twenty dollars."

In this case, since we are thinking about a particular shirt and can identify it, we would say:

• He encontrado una camisa que cuesta menos de veinte dólares.

On the other hand, if we have not yet found a shirt which we have identified, we might say the second sentence:

• "I want to find a shirt that costs less than twenty dollars."

Notice that in this case I don't have a particular one in mind. I haven't found one yet like I want. There could be several different shirts in this store, or in other stores, which would satisfy my need.

In Spanish we would have the following:

• Quiero encontrar una camisa que cueste menos de veinte dólares.