English Articles with Countable Nouns

Countable nouns can be preceded by determiners such as definite and indefinite articles. This usually happens with singular countable nouns, but the definite article "the" can also be used with plurals. 

Examples:

He saw a cat in a tree.
Tim and Kathy planted the trees in their backyard.
Roger picked up the empty can.

The first sentence has two countable nouns preceded by the indefinite article "a." The second example has the definite article "the" before a plural noun, and the last one has "the" before a singular noun.