uriga mudae uisangeul junbihal tenikka neoneun bunjangeul hae.

-ㄹ 테니까 means something like since I/we will..., because this is what I/we're going to do..., or so....

In this sentence, 우리가 무대 의상을 준비할 테니까 means:

• we’ll take care of preparing the stage costumes
• and that becomes the reason for telling the other person what to do next

This grammar is very common when the first clause is the speaker’s intention or plan, and the second clause is a request, suggestion, or command.

They are similar, but not identical.

• 준비하니까 = because we prepare / because we are preparing
• 준비할 테니까 = because we will prepare / since we’re going to prepare

The -ㄹ 테니까 form often sounds better when:

• the speaker is talking about a future plan
• the speaker is making a promise or commitment
• the next clause tells someone what to do

So here 준비할 테니까 fits naturally because it means we’ll handle that part, so you do this other part.

가 is the subject marker, so 우리가 means we are the ones doing the preparing.

Here, 우리가 helps identify the subject of the first action:

• 우리가 무대 의상을 준비하다 = we prepare the stage costumes

If you said 우리는, it would make we the topic instead. That can also be possible in some contexts, but 우리가 works well here because it clearly sets up who is doing that task.

So the sentence feels like:

• we will prepare the costumes
• you do the makeup

The 는 in 너는 adds a contrastive topic feeling.

So 너는 분장을 해 is not just you do the makeup. It feels more like:

• as for you, do the makeup
• you, on the other hand, do the makeup
• your part is the makeup

This matches the first clause nicely:

• we will prepare the costumes
• you do the makeup

That contrast is one of the main reasons 는 is used here.

They are included here for clarity and contrast.

Yes, Korean often omits subjects when they are obvious. But in this sentence, the speaker is dividing up responsibilities, so saying both sides makes the assignment very clear:

• 우리가 = our team / we
• 너는 = you

If the pronouns were omitted, the sentence could still be understandable from context, but it would lose some of that clear division of labor feeling.

무대 의상 is a noun phrase:

• 무대 = stage
• 의상 = costume / outfit

So 무대 의상 means stage costume or costume for a performance.

It refers to the clothes worn for a show, play, concert, or similar performance context.

This is an important vocabulary difference.

• 화장 = everyday makeup, cosmetics, putting on makeup in the normal sense
• 분장 = theatrical makeup, character makeup, disguise, makeup for a role

Because the context is clearly a performance setting with 무대 의상 (stage costumes), 분장 is the natural word. It suggests makeup done for the stage or for a specific character, not just ordinary beauty makeup.

Because 분장 can function as a noun, and Korean very often says noun + 을/를 하다 for activities.

So:

• 분장을 하다 = to do the makeup / to do the stage makeup
• 청소를 하다 = to do cleaning
• 준비를 하다 = to do preparation

Here, 분장 is the thing being done, so it takes the object marker 을.

You may also hear 분장해, and that is also natural in many situations. But 분장을 해 sounds very clear and task-oriented, as if naming the job itself: do the makeup.

Yes, it functions as a casual command or directive: do the makeup.

The ending 해 is plain casual speech, and 너 is also casual. So this sentence is appropriate when speaking to:

• a close friend
• someone younger
• someone you normally use casual speech with

It can sound rude if said to:

• a stranger
• a superior
• someone older
• someone you should speak politely to

A more polite version would use a different subject expression and a polite ending, for example:

• 우리가 무대 의상을 준비할 테니까 분장을 하세요.

It sounds like the speaker is assigning roles before a performance.

The overall feeling is:

• we’ll handle the stage costumes
• you handle the makeup

So it has a practical division of tasks nuance. Because of -ㄹ 테니까 and 너는, it does not sound like two unrelated actions. It sounds like a coordinated plan: we’ll do our part, so you do your part.