Learn Hardcore German: Erfahrung ist wichtig.

In German, when you make a general statement about a concept, you often drop the article entirely. Here Erfahrung is used in a generic sense (“experience in general”), so no der, die or eine appears.

What case and number is Erfahrung in this sentence?

Erfahrung is in the nominative singular.

Nominative because it’s the subject of the verb ist (“is”).
Singular because we’re referring to the idea of experience as a whole, not individual experiences.

What is the gender of Erfahrung and how would you say “an experience” or “the experience” in German?

Erfahrung is a feminine noun.

“The experience” = die Erfahrung
“An experience” = eine Erfahrung
Remember to adjust adjective endings or case endings if you put it into a different grammatical context.

Why isn’t wichtig declined here?

After the copular verb ist (a form of “to be”), adjectives remain in their base form—no endings. We call this a predicate adjective. It simply describes the subject without any case or gender agreement.

Could I use the plural Erfahrungen instead of the singular? If so, how would the sentence change?

Yes. If you want to emphasize individual experiences, use the plural:
Erfahrungen sind wichtig.
Here Erfahrungen (plural) takes the plural verb sind (“are”). The nuance shifts from “experience in general” to “specific experiences.”

Can you use the definite article for a general statement, e.g. Die Erfahrung ist wichtig?

You can, but it feels slightly more specific or abstract.

Die Erfahrung ist wichtig. might hint at “(this/that) experience is important” or “Experience (as a concept) is important” with a tad more emphasis.
Without the article (Erfahrung ist wichtig), it’s purely generic and neutral.