Learn Hardcore German: Der Stuhl ist klein.

In German, nouns each have a grammatical gender: masculine, feminine, or neuter. The word Stuhl (chair) is masculine, so its definite article is der.

Why do we say ist here, and not another form of sein?

The verb sein (to be) is irregular, and in the third-person singular (he, she, it), it becomes ist. Since Der Stuhl is third-person singular (it), we say ist.

Why is the adjective klein and not kleine?

When an adjective appears after sein (and similar verbs), it's used in the predicate position and does not take an ending. If the adjective were placed before the noun, as in Der kleine Stuhl, it would need an ending to agree with the noun in case, gender, and number.

Can I rearrange the sentence to something like Klein ist der Stuhl?

While Klein ist der Stuhl is grammatically correct, it sounds unusual or poetic in everyday German. The more natural way to say it is Der Stuhl ist klein.

Which case is Der Stuhl in?

Since Stuhl is the subject of the sentence, it’s in the nominative case. In German, the subject of a sentence with a form of sein stays in the nominative case.