Agen menandatangani kontrak setelah menerima uang muka.

It’s a derived verb built from:

• meN- (active verb prefix) + tanda tangan (signature) + -i (suffix) = menandatangani = to sign something.

Note the assimilation rule for meN-: when the base begins with t, the initial t is dropped. So meN- + tanda tangan → meN- + (t)anda tangan → menandatangani. The first t disappears; the t you still see is the one in tangan.

Related forms:

• penandatangan = signatory
• penandatanganan = the signing (event)
• Passive: ditandatangani = be signed (by)

With this root, -i is the standard, lexicalized choice: menandatangani [dokumen] = sign [a document]. The suffix -i often marks the target/location of an action.

You may see menandatangankan in some legal writing, usually meaning “to have (something) signed” or “to cause the signing,” e.g., arranging or executing a signing on someone’s behalf. For ordinary “sign a contract,” stick with menandatangani.

Colloquially, yes:

• Agen tanda tangan kontrak… (informal)
• Agen menandatangani kontrak… (neutral/formal)

There’s also a very colloquial synonym meneken (from Dutch tekenen): Agen meneken kontrak…

Indonesian has no articles. Bare nouns are fine. You can add a determiner if you need specificity:

• Indefinite: sebuah kontrak (a contract; emphasizes singular/new)
• Definite: kontrak itu / kontrak tersebut / kontraknya (that/the contract)

By default, the subject is controlled by the matrix subject, so it refers to the same subject as the main clause (agen). To avoid ambiguity, make it explicit:

• Setelah agen menerima uang muka, … Or use a passive subclause if the receiver doesn’t matter:
• Setelah uang muka diterima, agen menandatangani kontrak.

Yes. Both word orders are fine:

• Agen menandatangani kontrak setelah menerima uang muka.
• Setelah menerima uang muka, agen menandatangani kontrak. (use a comma when it comes first)

Indonesian doesn’t mark tense morphologically. Time is read from context and adverbs:

• Future: akan menandatangani (will sign), or rely on context.
• Completed: sudah/telah menandatangani (has signed).
• Not until: baru can add the nuance “only after”: Agen baru menandatangani kontrak setelah menerima uang muka.

Common passives:

• Short passive: Kontrak ditandatangani agen setelah agen menerima uang muka.
• With oleh: Kontrak ditandatangani oleh agen setelah ia menerima uang muka. If you don’t want to repeat the subject: Kontraknya akan ditandatangani setelah uang muka diterima.

The base is terima (receive), but as a finite verb you normally use the meN- form menerima. Bare terima survives in set phrases like terima kasih (thank you). You can also use mendapat (to get/obtain) with a slightly different nuance:

• setelah menerima uang muka (after accepting/receiving the down payment)
• setelah mendapat uang muka (after getting an advance)

Yes:

• tanda tangan is two words as a noun; but the verb menandatangani is one word.
• Don’t write tandatangan (as a noun) or menandatangi (missing “-ang-”).
• Passive is ditandatangani (not “di tandatangani”).

Context often suffices. If you need to mark plural explicitly:

• para agen (the agents; formal)
• agen-agen (reduplication)

Yes:

• Agen menandatangani kontrak sebelum menerima uang muka. Negation with a condition often uses sampai or sebelum:
• Agen tidak akan menandatangani kontrak sampai menerima uang muka.

Neutral–formal. A casual Jakarta-style version could be:

• Agen tanda tangan kontrak setelah terima DP. Even more casual: Agen bakal tanda tangan kontrak abis terima DP. (abis = setelah; very colloquial)

Use a causative or an agent phrase:

• Agen menandatangankan kontrak atas nama klien. (arranges/executes the signing on behalf of the client; legalese)
• More plainly: Agen menandatangani kontrak atas nama klien. (signs on the client’s behalf)