Rumpali vai kitaristi? Molemmat, koko bändi oli energinen.

Because vai is the normal word for or in a direct alternative question.

• Rumpali vai kitaristi? = Drummer or guitarist?

By contrast, tai is the general or used in statements and many non-question contexts.

• Hän on rumpali tai kitaristi. = He is a drummer or a guitarist.

A simple rule of thumb:

• vai = A or B?
• tai = ordinary or

Even though vai sets up a choice between two options, the reply can still be Molemmat if both are true.

They are in the nominative singular, which is the basic dictionary form.

In this kind of short either-or question, Finnish often just places the two options side by side:

• Rumpali vai kitaristi?

There is no article system in Finnish, so you do not need words for a or the.

A fuller version might be something like:

• Oliko se rumpali vai kitaristi?
• Kumpi se oli, rumpali vai kitaristi?

Even in a fuller sentence, rumpali and kitaristi would still normally stay in this basic form here.

Yes. Finnish often allows elliptical questions, especially in speech, dialogue, subtitles, and informal writing.

So Rumpali vai kitaristi? works the same way English can say:

• The drummer or the guitarist?

The missing material is understood from context. Finnish does this very naturally.

Molemmat means both.

Grammatically, it is a plural pronoun referring to the two options just mentioned. It can stand on its own, without repeating the noun.

So after Rumpali vai kitaristi?, the answer Molemmat means both of them.

If you made it a full sentence, you could say:

• Molemmat olivat energisiä. = Both were energetic.

In your sentence, Molemmat is a short answer first, and then the speaker adds more information.

Because koko and kaikki are used differently.

• koko bändi = the whole band / the entire band
• kaikki usually means all and is used with plural ideas

So these are natural:

• koko bändi
• kaikki jäsenet
• kaikki olivat energisiä

But kaikki bändi is not idiomatic Finnish.

A useful contrast:

• koko bändi = the band as one whole group
• kaikki bändin jäsenet = all the members of the band

Because bändi is grammatically singular.

Even though a band contains several people in real life, the noun bändi is one singular collective noun, so Finnish uses:

• bändi oli
• bändi oli energinen

This is similar to English the band was energetic. Finnish normally treats group nouns this way.

If you wanted to focus on the individual people instead, you would use a plural subject, for example:

• Bändin jäsenet olivat energisiä.

The comma shows a pause and helps split the reply into two parts:

1. Molemmat = the direct answer
2. koko bändi oli energinen = extra explanation

So the rhythm is roughly:

• Both — actually, the whole band was energetic.

This is a compact, spoken-style way of answering. In other writing styles, you might also see:

• Molemmat. Koko bändi oli energinen.
• Molemmat – koko bändi oli energinen.

So the comma is not changing the core grammar very much; it mainly reflects how the speaker is packaging the answer.

It is a very common Finnish loanword from English band, adapted to Finnish spelling and pronunciation.

So:

• English band
• Finnish bändi

It is perfectly normal in everyday Finnish, especially when talking about pop, rock, and similar music.

There is also a more native/formal Finnish word:

• yhtye = band / musical group

So depending on style, you may hear either bändi or yhtye.

Yes. The original is short and natural, but Finnish could also make the same idea more explicit.

For example:

• Oliko energinen rumpali vai kitaristi?
• Molemmat. Koko bändi oli energinen.

Or:

• Kumpi oli energinen, rumpali vai kitaristi?
• Molemmat olivat energisiä — itse asiassa koko bändi oli energinen.

The original version is simply more compact and conversational.