-자: Let's — The Casual Propositive

-자 is plain Korean for let’s — we do this together. 우리말샘’s propositive entry sits in 해라할 자리: same speech level as -어라, different job.

Gloss for learners: casual partner of -읍시다. Same invitation energy; different ceiling.

Shape

Verb stem + -자:

가자.                 Let’s go.
먹자.                 Let’s eat.
영화 보자.            Let’s watch a movie.
응원하러 가자.        Let’s go cheer.

Concert-and-match plans live here among friends:

치킨 먹자.
다시 보자.
노래방 가자.

Let’s vs do it vs shall we

가자.         Let’s go. (plain)
갑시다.       Let’s go. (formal)
가라.         Go. (command to you)
갈까?         Shall we go?

Register

저희 시작합시다.      meeting / polite group
우리 시작하자.        friends
시작하세요.           directing someone politely (-으세요 / 해요)

-자 with the wrong person sounds childish or disrespectful. With the right person, it is the default glue of plans.

Other -자 traps

Dictionary -자 also covers connectives (“as soon as,” “as a cause”) and -자 하다 intention patterns. Those are not this sentence-final let’s. If a -자 is in the middle of a longer sentence, you are probably not in this slot.

Ask: am I proposing we act together in banmal? If yes, -자. If the room is formal, use -읍시다. If you are ordering only them, use -어라.