Learn Hardcore Indonesian: Ada sepuluh murid baru di kelas matematika. - There are ten new students in math class.

Ada indicates existence or presence—it's equivalent to “there is”/“there are” in English. Placing it at the start introduces what follows as the thing that exists.

Why is sepuluh placed before murid, even though I learned adjectives follow nouns in Indonesian?

Adjectives do follow nouns, but cardinal numbers always precede the nouns they modify. So you say sepuluh murid, while an adjective like “new” comes after: murid baru.

Why isn't murid made plural with reduplication, like murid-murid?

When you specify a number (e.g., sepuluh), that already shows how many there are, so there’s no need to pluralize the noun. sepuluh murid clearly means “ten students.”

Why does baru come after murid instead of before it?

In Indonesian, adjectives always follow the noun they modify. So murid baru means “new student(s),” with baru (“new”) placed after murid (“student”).

What’s the difference between murid and siswa? They both mean “student,” right?

Yes, both can mean “student.” Murid often emphasizes the student–teacher relationship and is used broadly, while siswa is more specific to school students and is common in formal education contexts. In everyday speech they’re often interchangeable.

Why are there no articles (“a” or “the”) before kelas matematika?

Indonesian does not have definite or indefinite articles. Context tells you whether something is specific or general, so you simply say kelas matematika for either “a math class” or “the math class.”

What does di do in di kelas matematika?

The preposition di marks location. di kelas matematika literally means “in/at the math class.”

Why is it kelas matematika and not kelas dari matematika to mean “class of math”?

Compound nouns in Indonesian are formed by placing one noun after another without a preposition. So kelas matematika is “math class.” Adding dari (“of”) would sound redundant or unnatural here.

Can I start the sentence with the location, like Di kelas matematika ada sepuluh murid baru?

Yes. You can front an adverbial phrase of place for emphasis:
Di kelas matematika ada sepuluh murid baru
still means “There are ten new students in the math class.”

Is Ada always required? Could I just say Sepuluh murid baru di kelas matematika?

Without Ada, that string is just a noun phrase and lacks the verb “there is/are.” To make a complete sentence expressing existence, you need Ada:
Ada sepuluh murid baru di kelas matematika.