Die orkes tree op.

Because optree is a separable verb in Afrikaans.

The full dictionary form is optree = to perform / to appear on stage.
But in a normal main clause, the verb splits:

• Die orkes tree op. = The orchestra performs.

Here:

• tree is the verb part that moves to the usual verb position
• op stays at the end

This is very common in Afrikaans with separable verbs.

Compare:

• Die orkes tree vanaand op.
• Wanneer tree die orkes op?

But in some other structures, it stays together as optree, for example:

• Ek weet dat die orkes optree.
• Die orkes wil optree.

Tree op means perform, appear, or go on stage.

It is often used for:

• musicians
• bands
• orchestras
• actors
• speakers

So Die orkes tree op means the orchestra is performing, not just that it is physically stepping somewhere.

On its own, tree can mean something like step or tread, but in tree op / optree, the meaning is idiomatic.

It is present tense.

Afrikaans often uses the present tense where English would use:

• The orchestra performs
• The orchestra is performing
• sometimes even The orchestra will perform, depending on context

So Die orkes tree op could mean:

• the orchestra performs
• the orchestra is performing

The exact English translation depends on context.

Usually, Afrikaans verbs do not change according to the subject the way English verbs sometimes do.

In English:

• I perform
• he performs

In Afrikaans, the verb usually stays the same:

• Ek tree op
• Hy tree op
• Die orkes tree op

So you do not add an ending like English -s.

Die here means the.

It is not pronounced like English die.
It is pronounced more like dee.

So:

• die orkes = the orchestra

This is a very common beginner point, because the spelling looks misleading to English speakers.

No. Afrikaans does not use grammatical gender the way languages like German or French do.

So:

• die is the normal definite article for nouns
• it does not mean masculine, feminine, or neuter

In other words, you do not need to learn noun genders for Afrikaans articles in the same way you would in some other languages.

A rough English-friendly pronunciation is:

dee OR-kes TREE-uh op

A few notes:

• die = dee
• orkes sounds roughly like OR-kes
• tree is not exactly like English tree; it is closer to TREE-uh
• op has a short o, roughly like op in stop without the st

In a subordinate clause, the separable verb usually comes back together as optree.

Example:

• Ek weet dat die orkes optree.

So this is a useful contrast:

• Main clause: Die orkes tree op.
• Subordinate clause: ... dat die orkes optree.

That is one of the key patterns to learn with separable verbs in Afrikaans.

You would usually use the Afrikaans double negative:

Die orkes tree nie op nie.

This literally has two nie words:

• one before the final part
• one at the end of the clause

That double negative pattern is normal in standard Afrikaans.

Yes, depending on the meaning you want.

For example:

• Die orkes speel. = The orchestra is playing.
• Die orkes tree op. = The orchestra is performing / appearing on stage.

So speel focuses more on playing music, while optree / tree op focuses more on performing as an act or event.