Tool
Which particle? 조사
Short answer: Most Korean particles have two shapes. Use the first after a consonant (책은, 책이, 책을) and the second after a vowel (사과는, 사과가, 사과를).
Type a word in Hangul, e.g. 학교.
Questions
When do I use 은 or 는?
은 after a word ending in a consonant (책은), 는 after a vowel (사과는). The same idea works for 이/가 and 을/를.
Why is it 서울로, not 서울으로?
(으)로 takes 으 after a consonant — except ㄹ: 서울로, 연필로, 집으로.
Why is it 제가, not 저가?
저, 나 and 너 change before 가: 제가, 내가, 네가. 누구 + 가 becomes 누가.