Learn Hardcore Japanese: tomodati ha taisetu desu.

Japanese often omits subjects when they’re understood from context. Here the statement “Friends are important” is a general truth, so there’s no need to say 私は or 僕は. If you want to specify “my friends,” you would say 私の友達は大切です without adding another 私は.

What is the function of は in 友達は?

は is the topic‐marker particle. It marks 友達 (“friends”) as the topic you’re talking about—literally “as for friends….” It does not directly translate to “is,” but signals what the sentence will comment on.

What part of speech is 大切, and why isn’t there a な before です?

大切 (たいせつ) is a な-adjective (sometimes called an adjectival noun). When linking a な-adjective to a copula (です/だ), you do not insert な. You only use な when the adjective directly modifies a noun (e.g. 大切な本 “an important book”).

What does です do, and why is it used here?

です is the polite copula (linking verb), similar to English “is/am/are” but with politeness built in. It completes the sentence in a polite register: “Friends are important.” In casual speech you’d swap it for (see next question).