Learn Hardcore German: Der Stuhl ist stabil.

In German every noun has a grammatical gender. Stuhl (chair) is a masculine noun, so it takes the masculine definite article der in the nominative case.

What case is Der Stuhl in, and how do I know?

It’s in the nominative case because it’s the subject of the sentence (the thing doing or being). In German the subject of a sentence usually appears in the nominative.

Why is the adjective stabil not changed to something like stabile or stabiler?

Because stabil here is a predicative adjective (it follows the linking verb ist). Predicative adjectives in German do not get endings. Only attributive adjectives (those directly in front of a noun) are declined.

What exactly is ist doing in this sentence?

Ist is the 3rd-person singular present–tense form of the verb sein (to be). It links the subject Der Stuhl to the adjective stabil, expressing a state of being (“The chair is stable.”).

How would the sentence change if I wanted to say “The chairs are stable”?

Pluralize both the noun and the verb:
Die Stühle sind stabil.

Die for plural definite article in nominative
Stühle is the plural of Stuhl
sind is the 3rd-person plural of sein

Can I use other words instead of stabil to convey a similar meaning?

Yes, common synonyms include:

robust (robust)
fest (firm/solid)
standfest (steady)

Each has a slightly different nuance, but all can describe a chair that won’t wobble or break easily.

How do you pronounce Stuhl and stabil?

Stuhl: [ʃtuːl] (like “shtool”)
stabil: [ʃtaˈbiːl] (like “shta-BEEL”)

The ʃ sound is like English “sh,” and -ie- in stabil is a long ee sound.

If I wanted to use stabil attributively (in front of a noun), how would it change?

You must decline it according to gender, case, and article. For example, with a strong article in nominative masculine:
Der stabile Stuhl.
Here stabile gets an -e ending because attributive adjectives take endings.