Họ diễn cùng nhau.

A simple word-by-word breakdown is:

• Họ = they
• diễn = act / perform
• cùng nhau = together, literally something like with one another

So the structure is basically:

• Họ
  • diễn
    • cùng nhau
• they
  • perform/act
    • together

Họ is a third-person plural pronoun, so it means they / them.

A few useful notes:

• It normally refers to people, not things.
• It does not show gender.
• It does not tell you whether the group is all male, all female, or mixed.

So họ is a very general they.

Vietnamese verbs usually do not change form for tense the way English verbs do.

So diễn stays diễn whether the action is present, past, or future. The time is usually understood from:

• context
• time words
• extra particles

For example:

• Họ diễn cùng nhau hôm nay. = They perform together today.
• Họ đã diễn cùng nhau rồi. = They performed together already.
• Họ sẽ diễn cùng nhau vào tuần sau. = They will perform together next week.

So in Họ diễn cùng nhau, the tense is neutral unless context tells you more.

In this sentence, diễn most naturally means to act or to perform.

Depending on context, it could refer to:

• acting in a play or film
• performing on stage
• more generally, appearing in a performance

So English might translate it as:

• They act together
• They perform together

The best choice depends on the situation.

Both are possible, but they are not always identical.

• diễn often suggests acting / performing
• biểu diễn more clearly means to give a performance / perform

So:

• Họ diễn cùng nhau can sound natural, especially if the context is theater, acting, or performance.
• Họ biểu diễn cùng nhau may sound a bit broader or more explicit about stage performance.

If you are talking specifically about actors, diễn is very natural.
If you mean musicians, dancers, or performers in general, biểu diễn is also very common.

Họ diễn just means They act / They perform.

Adding cùng nhau makes it clear that they do it together, as a group or alongside one another.

So the difference is:

• Họ diễn. = They perform.
• Họ diễn cùng nhau. = They perform together.

Without cùng nhau, the sentence does not emphasize the shared action.

Yes, Họ cùng diễn is also possible.

The two are close in meaning, but there is a small nuance:

• Họ cùng diễn = They both/all perform; it emphasizes that they do the action jointly.
• Họ diễn cùng nhau = They perform together; this feels a bit more explicit and natural when stressing mutual participation.

Very roughly:

• cùng often means jointly / also / together
• cùng nhau more clearly means together with one another

In many everyday contexts, both work.

In Vietnamese, adverbial or complement-like expressions often come after the verb.

So:

• diễn cùng nhau = perform together

This is normal Vietnamese word order.

English often puts together after the verb too, so this sentence is actually fairly easy to map:

• They perform together
• Họ diễn cùng nhau

Usually, you would say cùng nhau, not just nhau, in this sentence.

nhau often appears in expressions involving mutual action, but it typically needs a suitable structure around it. For example:

• giúp nhau = help each other
• yêu nhau = love each other

In Họ diễn cùng nhau, the natural phrase is cùng nhau.

So learners should treat cùng nhau as a common chunk meaning together.

Not necessarily.

It can mean:

• they perform in the same production
• they act together as co-stars
• they appear together in a show
• they work together in a performance context

The exact meaning depends on context.

For example:

• in a movie context, it might mean They acted together
• in a concert context, it might mean They performed together on stage

Yes. In colloquial Vietnamese, diễn can sometimes mean something like:

• to pretend
• to be fake
• to put on an act
• to act dramatic

For example, someone might say another person is diễn if they are being theatrical or insincere.

But in Họ diễn cùng nhau, the most natural reading is the normal one: they perform/act together.
Context decides whether diễn is neutral artistic performance or something more figurative.

A rough guide is:

• Họ: like haw with a short, heavy falling tone
• diễn: roughly zee-uhn or yee-uhn, depending on accent, with a rising-glottal type tone
• cùng: roughly koong with a low tone
• nhau: roughly nyow

A very rough English-style approximation might be:

• haw zee-uhn koong nyow

But pronunciation varies by region, especially for d / gi / r sounds.

Yes, Vietnamese often omits subjects when they are obvious from context.

So in conversation, if everyone already knows who is being discussed, someone might simply say:

• Diễn cùng nhau.

But as a full standalone sentence for learners, Họ diễn cùng nhau is clearer and more complete.

It is neutral and natural.

• Họ is standard.
• diễn is normal everyday Vietnamese.
• cùng nhau is also standard.

So the sentence is neither especially formal nor especially slangy. It works in ordinary speech and writing, depending on context.