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.

Ist is the 3rd person singular form of the verb sein (to be).

Because der Bart is:

• third person: not I or you
• singular: one beard

the verb must be ist.

A few forms of sein are:

• ich bin
• du bist
• er / sie / es ist
• wir sind
• ihr seid
• sie / Sie sind

Because lang is being used after the verb ist. In German, adjectives in this position are called predicate adjectives, and they do not take endings.

So:

• Der Bart ist lang. = correct

But when the adjective comes before the noun, it does take an ending:

• der lange Bart = the long beard

So the difference is:

• Der Bart ist lang. → full sentence
• der lange Bart → noun phrase, not a full sentence

This is normal German main-clause word order.

German main clauses usually have the finite verb in second position, and that is exactly what happens here:

• Der Bart = first position
• ist = second position
• lang = rest of the sentence

So this sentence is very straightforward.

Normally, you do need the article here.

• Der Bart ist lang. = normal
• Bart ist lang. = not normal standard German in this context

German usually requires an article with a singular countable noun like Bart, unless you are using a special style such as headlines, labels, or very unusual phrasing.

Yes, but it means something slightly different grammatically.

• Der Bart ist lang. = a complete sentence
• der lange Bart = a noun phrase meaning the long beard

So der lange Bart is not the same kind of structure. It names something; it does not by itself make a full statement.

Not in this sentence.

In Der Bart ist lang, lang is a predicate adjective, so it stays in its basic form.

But if the adjective comes before the noun, then it must agree and take an ending:

• der lange Bart

So:

• after sein → lang
• before the noun → lange

A rough pronunciation guide is:

• der ≈ dehr (with a German r)
• Bart ≈ bahrt
• ist ≈ ist
• lang ≈ lahng

A more German-like IPA version is:

[deːɐ̯ baʁt ɪst laŋ]

A useful point:

• lang ends with the ng sound, like in English song

Yes, it is grammatically natural and simple.

It is a very typical beginner sentence because it shows:

• a noun with an article: der Bart
• the verb sein: ist
• an adjective after the verb: lang

In real conversation, the sentence would usually need context — for example, whose beard you are talking about — but grammatically it is perfectly normal.