나는 and 난 are the same, 난 is just reduced to be easier to write/pronounce, kind of like a contraction in English. You wouldn't use it in formal writing or speaking, you'd use the full 나는 form (or more likely 저는) The difference between 나는 and 내가... well, technically, -는 is a topic marker and 가 is a grammatical subject marker. Korean is a topic-comment langauge, so the topic (or "main point") of the sentence is more important than the grammatical subject, which is just "who did the verb". The 는 *topic) is kind of like where in the sentence you'd choose to bang the nail in when putting it up on the wall - it's whatever the sentence is 'about'. The equivalent in English is the word in the sentence that you'd stress. It's a hard concept to wrap your head around. e.g. /I/ didn't eat the pizza - 나는 피자를 안 먹었다 (Here's a sentence about me: I didn't eat the pizza) I didn't eat the /pizza/ - 피자는 내가 안 먹었다 (Here's a sentence about the pizza: I didn't eat it) Nine times out of ten, in simple sentences, your grammatical subject (이/가) will be the same noun as your topic (은/는), so the 은/는 and the 이/가 will go on the same noun; in these cases, you only write/say the 은/는, as it takes priority, as mentioned above. The type of sentences that need BOTH 은/는 and 이/가 are ones in which the topic (main noun, which the sentence is about) is different from the grammatical subject. The second one above, which has pizza as the topic but 나 as the grammatical subject is one type, but there are others. Consider 나는 피자가 좋다 - I like pizza. Note that 나 is the topic (stressed/emphasised word), i.e. it would be used in a comparison like /I/ like pizza but /my sister/ likes hot dogs In contrast, if pizza was the topic, the sentence could be part of a comparison like: I like /pizza/ but I don't like /chicken/ (피자는 좋은데 치킨은 싫다) - literally, pizza's good but chicken sucks. 'I' (나) doesn't appear in this sentence - that's how Korean works with comparative 'I like...' sentences hope that helps!