Questions & Answers about Der Bart ist lang.
Because Bart is a masculine noun in German. German nouns have grammatical gender, and the dictionary form is der Bart.
In this sentence, Bart is singular and in the nominative case, so the correct article is der.
In German, all nouns are capitalized. So Bart must always be written with a capital B.
That is one of the big spelling differences from English.
It is in the nominative case because it is the subject of the sentence — the thing being described.
In Der Bart ist lang, the beard is what is long, so der Bart is the subject.