Jika kamu lupa membawa kartu identitas, maka kamu harus mengisi formulir lagi.

Jika introduces a conditional clause (if-clause). It’s often slightly more formal/written than kalau, but in many everyday situations they’re interchangeable:

• Jika kamu lupa... = Kalau kamu lupa...
  In more formal announcements/instructions, jika is very common.

Maka roughly signals then and marks the result clause more explicitly: If X, then Y.
It’s optional in many cases:

• Jika kamu lupa membawa kartu identitas, kamu harus mengisi formulir lagi. (still correct) Using maka can make the cause→effect structure feel clearer or more formal.

Indonesian often repeats the subject for clarity, especially when the first clause is long. You can omit the second kamu if it’s clear:

• Jika kamu lupa membawa kartu identitas, (maka) harus mengisi formulir lagi.
  However, that version can sound a bit more “instruction-like” and slightly less natural in some contexts. Repeating kamu is very normal.

Kamu is informal and is used with friends, family, peers, or people younger than you (depending on region and relationship). Alternatives:

• Anda (neutral/formal, customer service, public notices)
• Bapak/Ibu (polite, addressing an adult directly)
• Omitting the pronoun (common in instructions): Jika lupa membawa kartu identitas, harap isi formulir lagi.
  So in an official setting you might see Jika Anda lupa... maka Anda harus...

This is a common pattern: lupa + (verb) = forget to (do something).
Here, membawa = to bring, so:

• lupa membawa kartu identitas = forgot to bring an ID card
  You’ll see similar structures like lupa bayar (forgot to pay) or lupa menutup pintu (forgot to close the door).

They express different “types” of forgetting:

• lupa membawa X = forgot to bring X (a missed action)
• lupa tentang X / lupa akan X = forgot about X (forgot the topic/thing itself)
• lupa dengan X is used in some regions/colloquial speech, but lupa tentang/akan is more standard.

Kartu identitas is a general term: identity card/ID card (can include many kinds of IDs).
KTP (Kartu Tanda Penduduk) is a specific Indonesian national ID for citizens. In casual contexts people might say:

• lupa bawa KTP
  In more general or formal wording, kartu identitas is safer.

Harus expresses obligation/necessity: must / have to. It’s fairly strong.
If you want softer:

• perlu = need to (often less forceful)
• sebaiknya = should / it’s better to
  So kamu harus mengisi formulir lagi is “You must fill out the form again.”

Indonesian often omits articles like a/the. Mengisi formulir is a natural, generic phrase meaning fill out the form (the relevant form in that situation).
You can add a classifier for emphasis or specificity:

• mengisi sebuah formulir = fill out a (certain) form
  But it’s not required and may sound slightly more deliberate.

Here lagi means again: you must fill out the form again (repeat the action).
Lagi can also mean more/another depending on context:

• minta lagi = ask again / ask for more
  In this sentence, the “repeat” meaning is the natural one.

Yes. Indonesian allows flexible ordering, especially with conditionals:

• Kamu harus mengisi formulir lagi jika kamu lupa membawa kartu identitas.
  This keeps the same meaning and is common in speech and writing.

A comma is very common when an if-clause comes first, especially in writing:

• Jika ..., maka ...
  In casual writing you might see it without a comma, but the comma improves readability and is generally recommended.