이/가 is added to the end of a subject to designate it as the subject of the sentence. For words ending a vowel, 가 is added, and for words ending in a consonant, 이 is added.
이/가 also functions to particularly emphasize the preceding subject.
It is used to express new information in a sentence, that is, the introduction of a new topic.

Details

When 가 is added to 나, 저 and 누구 they combine as follows:

나 + 가 → 내가

내가 리처드예요.

I'm Richard.

나가 리처드예요. ✕

저 + 가 → 제가

제가 할게요.

I'll do it.

저가 할게요. ✕

누구 + 가 → 누가

누가 청소하겠어요?

Who will clean up?

누구가 청소하겠어요? ✕

Mark the subject of the sentence.

Details

Usage

Emphasis

The particle 이/가 can also be used to emphasize the preceding subject.

제가 할게요.

I will do it.

옛날 어느 마을에 착한 농부가 살았다. 농부는 아직 결혼하지 않고 혼자 살았다.

Once upon a time, in a village, there lived a kind farmer. The farmer was still unmarried and lived alone.

When the particle 가 is combined with 나, 너, 저, 누구 or 무엇 a phonetic transformation takes place.

In honorific language, 이/가 (Subject) is replaced by 께서 (Subject).