Here's why:
* Noun: A noun is a word that names a person, place, thing, or idea.
* Declarative sentence: A declarative sentence makes a statement.
In this sentence:
* "The two boys" is the subject (the people or things being talked about). This phrase is a noun phrase, which is a group of words that function as a noun.
* "are tall" is the predicate (the part that says something about the subject).
Let me know if you have any other questions!