The one rule behind most particles
Many particles have two forms. Which one you use depends only on whether the noun ends in a final consonant (받침):
| Particle | After a consonant | After a vowel |
|---|---|---|
| Topic | 책은 | 친구는 |
| Subject | 책이 | 친구가 |
| Object | 책을 | 친구를 |
| And (written) | 책과 | 친구와 |
| To / by means of | 책으로 | 친구로 · 서울로 (ㄹ) |
| "It is…" | 책이에요 | 친구예요 |
Notice the exception: nouns ending in ㄹ take 로, not 으로 (서울로, 지하철로).
은/는 or 이/가?
Both can mark what a sentence is about, but they feel different. 은/는 sets the topic or contrasts ("As for me…"). 이/가 points out new information or answers "who/what?" (누가 왔어요? 민수가 왔어요.) Start with 은/는 to introduce yourself and 이/가 with 있어요 / 없어요.
Special forms
나, 저, 너 and 누구 change with 가: 내가, 제가, 네가, 누가.
Numbers
For numbers, the rule follows how the number is read in Sino-Korean: 3 (삼) ends in ㅁ, so 3은; 2 (이) ends in a vowel, so 2는.
FAQ
What about English words or names?
Use how the word sounds in Korean. "Tom" (톰) ends in a consonant: 톰은, 톰이. "Anna" (안나) ends in a vowel: 안나는. The tool guesses from the last letter, so writing the name in Hangul gives the best result.
Is 하고 the same as 와/과?
Yes, both mean "and/with". 하고 is common in speech and does not change form. 와/과 is more written. 이랑/랑 is casual.