Learn Hardcore Korean: chingudeuri hakgyoe isseoyo.

The suffix -들 makes 친구 (“friend”) plural, so 친구들 means “friends.” In Korean, you often use -들 to clarify that you’re talking about more than one person or thing, though in some contexts Korean leaves plurals implicit.

Why do we add the particle -이 after 친구들 (making 친구들이)?

The particle -이 is the subject marker for nouns ending in a consonant. It tells us that 친구들 (“friends”) is the subject of the sentence. If the noun ends in a vowel, you’d use -가 instead.

What does the particle -에 do in 학교에 있어요?

The particle -에 marks a static location when used with verbs of existence like 있다. Here, 학교에 means “at school” or “to school” in contexts of location. Combined with 있어요 (“exist”/“be located”), 학교에 있어요 means “(they) are at school.”

What exactly does 있어요 mean in this sentence?

있어요 is the polite present form of 있다, which here means “to exist” or “to be located.” So 친구들이 학교에 있어요 literally means “Friends exist at school,” i.e. “My/Their friends are at school.”

Can 있다 also mean “to have”?

Yes. When you want to say “I have something,” you use (object) + 이/가 + 있다. For example, 돈이 있어요 means “I have money.” Context tells you whether 있다 means “to exist” (location) or “to have.”

Why is the polite ending -요 used, and can I omit it?

The -요 ending makes the sentence polite but informal—suitable for strangers or people you’re not close with. In very casual speech (among close friends), you could drop -요 and say 친구들이 학교에 있어. In more formal or honorific structures, you might use 있습니다 or 계세요.

Could I use a topic marker like 친구들은 instead of 친구들이?

Yes. 친구들은 학교에 있어요 shifts focus to “as for friends…” often implying a contrast (e.g., “My friends are at school, but my family is at home”). -는/은 is the topic marker, -이/가 is the pure subject marker. Which you choose depends on what nuance or emphasis you want.