Пока неизвестно, кто придёт.

who

Breakdown of Пока неизвестно, кто придёт.

• прийти: to come
• пока: still
• неизвестно: it is unknown

Questions & Answers about Пока неизвестно, кто придёт.

What does пока mean here? Does it mean while?

Here пока means for now / so far / at the moment, not while.

So:

• Пока неизвестно = It’s not known yet / So far it’s unknown

This is a very common use of пока in everyday Russian.

Compare:

• Пока неизвестно, кто придёт. = It’s not known yet who will come.
• Пока я работаю, он читает. = While I’m working, he’s reading.

Same word, different meaning depending on context.

Why is there no word for it is in the sentence?

Russian usually omits the present-tense form of "to be".

So where English says:

• It is unknown

Russian simply says:

• неизвестно

Literally, the sentence is closer to:

• For now unknown, who will come

but natural English is:

• It is not yet known who will come

This is normal Russian grammar, not an incomplete sentence.

Why is it неизвестно and not неизвестен / неизвестна / неизвестны?