Learn Hardcore Indonesian: Pesta itu untuk merayakan ulang tahun teman saya. - That party is to celebrate my friend’s birthday.

Pesta means “party.” Itu is a demonstrative meaning “that” or simply “the” in context. In Indonesian, demonstratives normally follow the noun they modify. So pesta itu translates as “the party” or “that party.”

Why is untuk used before merayakan, and can we drop it?

Untuk introduces purpose, equivalent to “in order to” or “for” in English. Here it marks why the party is held:
• Pesta itu untuk merayakan… = “The party is for celebrating…”
If you drop untuk, you’d get Pesta itu merayakan…, which literally means “The party celebrates…,” sounding awkward because events don’t actively celebrate without a purpose marker.

What does merayakan mean, and why not use a different form like dirayakan?

Merayakan is the active verb “to celebrate (something).” It pairs naturally with untuk to express purpose.
• Dirayakan is the passive “to be celebrated.” If you said Pesta itu dirayakan…, you’d be saying “That party is being celebrated…,” which reverses the logic: we celebrate an occasion by holding a party, not celebrate the party itself.

What is ulang tahun literally, and how did it come to mean “birthday”?

Literally: ulang = “repeat,” tahun = “year.” Together, ulang tahun is “(the) repetition of a year,” i.e. an anniversary of someone’s birth—hence “birthday.”

Why is it teman saya and not temanku?

Both mean “my friend.”
• Teman saya uses a separate possessive pronoun saya, a more neutral or formal style.
• Temanku attaches the possessive suffix -ku directly to teman, a bit more casual. Both are correct; choice depends on register.

Can we replace untuk with buat in this sentence?

Yes. Buat is an informal synonym of untuk. In casual speech you might hear:
• Pesta itu buat merayakan ulang tahun teman saya.
In formal writing or speech, stick with untuk.

Is it okay to say Pesta itu diadakan untuk merayakan ulang tahun teman saya instead?

Absolutely. Diadakan means “is held” or “is organized.” Adding it makes explicit who arranges the event:
• Pesta itu diadakan untuk merayakan ulang tahun teman saya.
= “That party is held to celebrate my friend’s birthday.”

Could we reorder the phrase as Pesta itu merayakan ulang tahun teman saya?

Literally you could say that, but it sounds odd. It implies the party itself is doing the celebrating. Native speakers prefer to show purpose with untuk or diadakan untuk, because you “hold a party to celebrate” rather than “a party celebrates.”