Learn Hardcore Malay: Jika borang tidak sah, kita perlu mengisi borang baharu di kaunter sempadan. - If the form is not valid, we need to fill in a new form at the border counter.

Jika is a formal conjunction meaning if. It’s common in written or official contexts. In everyday speech, speakers often substitute kalau for the same purpose.
Example:

Jika hujan, kelas dibatalkan. (formal/written)
Kalau hujan, kelas dibatalkan. (informal/spoken)

What does sah mean, and why do we say tidak sah?

Sah means valid, legitimate, or officially recognized. Adding tidak (not) gives tidak sah, which means invalid. This is typical in administrative or legal contexts. You might also encounter batal (void), but tidak sah is the neutral way to say that something isn’t valid.

Why is the verb mengisi used instead of isi?

Mengisi is the standard active verb formed with the prefix meN- and the suffix -i, meaning to fill in. By contrast, isi on its own can be a noun (content) or an informal verb/imperative. In formal Malay you say mengisi borang (“to fill in the form”). In casual spoken Malay you might hear isi borang.

What’s the difference between baru and baharu?

Both words mean new, but:

Baru is the everyday term for new.
Baharu is more formal or literary, often implying something just issued or completely brand-new.
In an official context (replacing an invalid form), borang baharu sounds more appropriate than borang baru.

Why is di placed before kaunter sempadan, and how should it be written?

Di is the preposition for location, meaning at (or in). It always precedes the noun phrase:
di kaunter sempadan = at the border counter.
Modern Malay orthography writes di as a separate word (no hyphen, no attachment).

Why does the sentence use kita instead of kami or anda?

Kita is an inclusive we (includes speaker and listener).
Kami is an exclusive we (excludes listener).
Anda means you (formal singular or plural).
Using kita makes the instruction apply to all of us (e.g., travellers and officers together). If you want to address a single person directly, you might use anda instead.

What does perlu mean, and are there other ways to express need to in Malay?

Perlu means need to or must in a neutral, formal sense. Synonyms include:

Harus (formal, similar to perlu)
Kena (colloquial, common in spoken Malay)
Wajib (stronger, meaning obligatory)
Examples:
Anda perlu mengisi borang.
Anda harus mengisi borang.
Anda kena mengisi borang.
Anda wajib mengisi borang.

Why is there a comma after sah, and can the if clause be moved to the end?

When a conditional clause (the if clause) appears at the beginning of a sentence in written Malay, it’s customary to add a comma before the main clause. You can also place the condition at the end:
Kita perlu mengisi borang baharu di kaunter sempadan jika borang tidak sah.
In that order, the comma is optional, especially in informal writing.