Learn Hardcore Indonesian: Setelah perbaikan selesai, teknisi mengecek kebocoran lagi. - After the repair is finished, the technician checks the leak again.

Both setelah and sesudah mean after. They are near-perfect synonyms; setelah is slightly more common in everyday and written Indonesian. You can say Setelah perbaikan selesai or Sesudah perbaikan selesai with no change in meaning.

In perbaikan selesai, what is selesai exactly? Is it a verb or an adjective?

Selesai functions like an adjective meaning finished/done, but it often serves as the predicate of a clause: perbaikan selesai = the repair is finished. You could add aspect markers: perbaikan sudah/telah selesai to emphasize completion. If you want a verb that takes an object, use menyelesaikan (to finish something).

Could I say Setelah sudah selesai perbaikan instead?

That word order is unnatural. Use either:

Setelah perbaikan selesai, …
Setelah perbaikan sudah/telah selesai, …
Setelah selesai diperbaiki, … (after it was repaired) Putting sudah before selesai is fine, but keep perbaikan before selesai in this structure.

Why is there a comma after the first part? Is it required?

It’s standard to put a comma when a subordinate clause comes first: Setelah …, [main clause]. In casual texting you might see it omitted, but in careful writing the comma improves clarity and is recommended.

What does teknisi mean? Is there a more informal word?

Teknisi means technician and is neutral/formal. In some contexts (especially automotive) people also say montir (mechanic), which is more colloquial and specific. For formal writing, teknisi is safest.

How is mengecek formed from cek, and is it the best choice here?

Mengecek comes from cek with the prefix meN-; because cek is monosyllabic, it becomes mengecek. It means to check. A more formal synonym is memeriksa. All are fine here:

teknisi mengecek kebocoran
teknisi memeriksa kebocoran

Does kebocoran mean a leak or leakage? How is it different from bocor?

Bocor is a base word meaning leaking/leaky (adjective/intransitive verb). Kebocoran is a noun (formed with ke- -an) meaning leak/leakage. With action verbs you’ll typically use the noun: memeriksa kebocoran (check for leaks). If you describe a state: pipa itu bocor (that pipe is leaking).

Does lagi here mean again or still? How would I say still?

Here lagi means again. To say still, use masih: teknisi masih mengecek (the technician is still checking). Don’t use lagi for still in standard Indonesian.

Where can I place lagi or its alternatives? Are there more formal options?

Neutral: place lagi after the verb phrase: mengecek kebocoran lagi.
More formal/journalistic: kembali, often before the verb: teknisi kembali memeriksa kebocoran; or after the verb: memeriksa kembali (a fixed, common collocation).
Re- as in redo: use -ulang after the verb: cek ulang / periksa ulang kebocoran. All three are natural; pick by register.

Could teknisi mengecek kebocoran lagi mean “another leak” rather than “check again”?

In this sentence it’s read as check again. To say another leak, you’d normally change the structure: Ada kebocoran lagi (there’s another leak) or kebocoran lain (a different/another leak). If you fear ambiguity, make it explicit: teknisi mengecek lagi apakah ada kebocoran.

How would I say After finishing the repair, the technician checked for leaks again using a verb rather than a noun for repair?

Use selesai + verb or a passive:

Setelah selesai memperbaiki, teknisi mengecek kebocoran lagi.
Passive focus on the thing repaired: Setelah selesai diperbaiki, teknisi mengecek kebocoran lagi.

Is tense marked anywhere here? How do we know it’s past?

Indonesian doesn’t mark past tense on the verb. Time is inferred from context markers like setelah (after), time adverbs (kemarin, tadi), or aspect words (sudah/telah). The setelah clause already signals a completed sequence.

Can I make it passive?

Yes. For example:

Setelah perbaikan selesai, kebocoran dicek lagi oleh teknisi.
Or with a neutral passive: Setelah perbaikan selesai, kebocoran kembali diperiksa. Passive is common in reports and formal texts.

Is mengecek too informal for formal writing?

Mengecek is widely used, but in formal or technical writing memeriksa, memeriksa kembali, or the noun pengecekan often sound more polished:

Setelah perbaikan selesai, dilakukan pengecekan kebocoran kembali.

How do I indicate a specific or indefinite technician (the vs a)?

Indonesian has no articles. Context supplies definiteness. If you want to say a technician, use seorang teknisi. For plural technicians, use para teknisi or beberapa teknisi.

Are there good colloquial alternatives?

Yes, in casual speech you might hear:

Habis perbaikannya beres, teknisi ngecek bocor lagi. Here habis = after (colloquial), beres = finished (colloquial), ngecek = check (informal), bocor used as a shorthand for kebocoran.

Any subtlety between lagi and kembali?

Both mean again, but:

Lagi is neutral and very common in speech.
Kembali feels a bit more formal/journalistic and often comes before the verb (kembali memeriksa). With some verbs, [verb] kembali is a set phrase (e.g., memeriksa kembali) meaning re-check/review.