Learn Hardcore Finnish: Onko sinulla aikaa nyt?

Onko is on (is/exists) + the question clitic -ko/-kö, so it literally means is it / is there? In Finnish, yes/no questions are often formed by attaching -ko/-kö to the word you want to “question,” and that word typically comes first.
So Onko sinulla aikaa nyt? is literally something like Is there (for you) time now?

What exactly is -ko/-kö, and why is it -ko here?

-ko/-kö is a yes/no question marker. It attaches to a word (most often the verb) to turn the whole sentence into a yes/no question.
It follows vowel harmony:

Use -ko after back vowels (a, o, u) → on + ko = onko
Use -kö after front vowels (ä, ö, y) → e.g. tuleeko vs tuleekö depends on the stem’s vowels (common example: onko vs menetkö)

Why is it sinulla instead of a word meaning “you” like sinä?

sinulla is sinä (you) in the adessive case (-lla/-llä), which often expresses “at/on/by someone.”
Finnish commonly expresses possession or “having” with the structure:

X-lla on Y = X has Y (literally At X is Y)
So:
sinulla on aikaa = you have time (literally at you is time)

Is sinulla always required, or can it be omitted?

It can be omitted if the context is clear, but it’s often kept for clarity. For example:

Onko sinulla aikaa? = explicitly Do you have time?
Onko aikaa? = Is there time? / Do we have time? (more general; could mean “Does anyone have time?” or “Is there time available?”)

Why is aikaa not aika?

aikaa is the partitive singular of aika (time). After existential/possessive on-sentences, Finnish often uses the partitive to express an indefinite amount:

sinulla on aikaa = you have (some) time / time available
Using aika (nominative) would sound like you mean a specific, complete unit of time in a more “counted/whole” way, which is not the usual idea here.

How do I know aikaa is partitive, and what does partitive “feel like” here?

The partitive singular ending often appears as -a/-ä (or -ta/-tä, etc.), and aika → aikaa is a common pattern where the stem lengthens.
The “feel” here is:

partitive = an unspecified quantity, “some,” “any,” or “available amount”
So Onko sinulla aikaa? is about availability, not a particular scheduled block.

What does nyt do, and can it move around?

nyt means now and sets the time frame. Finnish word order is fairly flexible, and moving nyt changes emphasis:

Onko sinulla aikaa nyt? = neutral: do you have time now (as opposed to later)?
Onko sinulla nyt aikaa? = emphasizes now (right at this moment)
Nyt, onko sinulla aikaa? = Now, do you have time? (discourse-style lead-in)

Could I say Onko sinulla nyt aikaa? instead—does it mean the same thing?

Yes, it’s very close in meaning. The difference is emphasis:

Onko sinulla aikaa nyt? puts aikaa earlier and feels slightly more neutral.
Onko sinulla nyt aikaa? highlights nyt more strongly: “Do you have time right now?”

How would this change if I’m being formal or addressing more than one person?

You’d use teillä (adessive of te, formal you or plural you):

Onko teillä aikaa nyt? = Do you (formal/plural) have time now?
Informal singular is sinulla as in the original sentence.

How do I answer this question in Finnish?

Common short answers:

On. = Yes. (literally “Is.”)
Ei ole. = No. (literally “Is not.”)
More explicit:
On (minulla) aikaa. = I have time.
Ei ole (minulla) aikaa. = I don’t have time.

How would I make the negative question: “Don’t you have time now?”

Use Eikö (negative + question clitic):

Eikö sinulla ole aikaa nyt? = Don’t you have time now? / Haven’t you got time now?
Here ole is the negative form of on used with ei-negation.

Are there common spoken/colloquial versions of this sentence?

Yes. In casual speech, sinulla often becomes sulla, and minulla becomes mulla:

Onks sulla aikaa nyt? (very common)
Here Onks is a spoken contraction of Onko.