-자

-자 is the casual way to say "let's (do something)" to close friends or people younger than you.

문장 끝·말투Endings & speech style

Han: Negative suggestions are made with -지 말다, so "let's not" is -지 말자, not 안 + verb + 자.

Examples

내일 같이 영화 보러 가자.Let's go see a movie together tomorrow.

우리 커피 한잔 마시자.Let's have a cup of coffee.

비가 오니까 오늘은 나가지 말자.It's raining, so let's not go out today.

배고파. 우리 밥 먹자!I'm hungry. Let's eat!

-자vs-(으)ㄹ까?Asks the listener's opinion ("shall we?"), so it sounds softer than the direct proposal -자: 같이 갈까? vs 같이 가자.

-자vs-아/어요 (suggestion)The polite-speech way to say "let's" to elders or people you are not close to: 선생님, 같이 가요.

V (action verb stem)+자

가다 → 가자, 먹다 → 먹자

It is casual speech (반말): use it with close friends or younger people. With older people or strangers, make the suggestion with -아/어요 instead (같이 가요).
For "let's not", use -지 말자 (가지 말자), not 안 + verb + 자.
It attaches to action verbs; with adjectives it usually sounds wrong, so change the expression: 조용히 하자, not 조용하자.
No past tense: attach 자 to the bare stem (먹자). -았/었자 is a different grammar meaning "even if".

Wrong: ~~비가 오니까 오늘은 안 나가자.~~

Right: 비가 오니까 오늘은 나가지 말자.

Negative suggestions are made with -지 말다, so "let's not" is -지 말자, not 안 + verb + 자.

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