이고 · 이라서 · 이니까 · 이면: Linking with the Copula

You already know how to end a sentence with the copula — 학생이에요 ("I'm a student"). But most real speech doesn't stop at one clause; it links. "I'm a student and he's a teacher," "because it's the weekend," "if it's Busan." Korean builds all of these by giving the copula 이다 a connective ending instead of a sentence-final one, and the same simple rule you met on day one still governs the shape: keep 이 after a consonant, drop it after a vowel. This page covers the four you will use every day — 이고, 이라서, 이니까, and 이면 — and shows that the copula quietly reuses a contrast you already met on verbs.

The one thing to fix before anything else: the copula connects on its 이 stem, not on the whole word 이다. You never say ×책이다고 or ×학생이다서. The 다 is only the citation/sentence-final ending; when the copula links, 다 disappears and the ending attaches to 이.

이고 — "and" (linking predicates)

To say "A is X and B is Y," attach 고 to the copula: 이고 after a consonant, plain 고 after a vowel (the 이 drops). This is the same coordinating 고 that action verbs use (먹고 자요, "eat and sleep"), simply riding on the copula.

Notice 가수 ends in a vowel, so we get 가수고, but 책 ends in a consonant, so we get 책이고. That single batchim test is the whole rule.

이라서 — "because it is" (plain reason)

To give a reason built on a noun predicate — "because it is a/an X" — use 이라서 after a consonant, 라서 after a vowel. This is the copula's version of the verb ending 아서/어서 ("because"), but the copula is irregular here: it does not take a 하다-style shape. The everyday form is 이라서 (the fuller 이어서 also occurs in careful speech and writing).

Again the vowel-final noun 가수 sheds the 이: 가수라서, not ×가수이라서.

이니까 — "since it is" (reason, and it can license a command)

이니까 (consonant) / 니까 (vowel) also means "because / since it is," and here is where the copula reveals something structural: 이라서 and 이니까 inherit the very same division of labour that separates 아서 from (으)니까 on verbs. Both give reasons, but 니까 foregrounds the reason as grounds the listener should act on, while 아서/이라서 states a plainer, more matter-of-fact cause.

The practical consequence is a hard rule you can lean on: only 이니까 can head an order or a suggestion; 이라서 cannot. You may say 주말이니까 쉬세요 ("it's the weekend, so rest"), but 주말이라서 쉬세요 is ungrammatical — 이라서 can't stand in front of a command. Reserve 이라서 for plain statements:

• ✅ 주말이니까 쉬세요. — reason + command → 니까 is required.
• ❌ 주말이라서 쉬세요. — 이라서 cannot introduce a command or suggestion.
• ✅ 주말이라서 좀 피곤해요. — 이라서 with a plain statement is fine.

Because this is exactly the 아서-vs-니까 contrast you already meet on verbs, you don't have to learn a new rule for the copula — you just apply the one you know. The full breakdown lives on the 아서 vs 니까 page.

이면 — "if it is"

To build a conditional on a noun predicate — "if it is an X" — use 이면 after a consonant, 면 after a vowel. This is the copula wearing the conditional ending (으)면.

부자 (a vowel-final noun) gives 부자면; 부산 and 만원 (consonant-final) give 부산이면, 만원이면.

The four endings at a glance

Why the copula "is a verb" matters here

If you have read that Korean adjectives are really descriptive verbs, the same insight explains why 이다 connects at all: 이다 conjugates like a predicate, not like a noun, so it can take the ordinary clause-linking endings 고, (으)니까, (으)면. The only genuinely irregular member is the reason ending — the copula uses 이라서 (an 라-stem form), never a ×이하서 or a bare ×이서. That 라 irregularity is the same one that resurfaces in the copula's quotation forms (이라고, 이라는); see 이라고 · 이라는 · 이란. Everywhere else, 이다 simply slots into the endings you already use on 먹다 and 좋다.

Common Mistakes

1. Leaving 이다 whole before the ending. The copula drops 다 and connects on 이. This is the number-one English-speaker error, because English keeps "is" intact ("it is a book and…").

2. Forgetting to drop 이 after a vowel. After a vowel-final noun the 이 disappears.

3. Using 이라서 in front of a command or suggestion. Only 이니까 can do that.

4. Leaving 다 in before the reason ending. Exactly as with 이고, the copula sheds 다 and builds "because" on its 이 stem — ×학생이다서 is never right.

Key Takeaways

• All four endings attach to the copula's 이 stem (drop the 다): 이고, 이라서, 이니까, 이면.
• The batchim test decides the shape: consonant → keep 이; vowel → drop it (가수고, 가수라서, 의사니까, 부자면).
• 이라서 and 이니까 both mean "because it is," but they split labour exactly like verb-level 아서 vs 니까: only 이니까 can precede a command or suggestion.
• The copula's reason form is irregular — 이라서 (an 라-stem), the same 라 that shows up in its quotation forms 이라고 / 이라는.
• Never leave 이다 whole before a connective ending: ×책이다고 → 책이고, ×학생이다서 → 학생이라서.