Odotan sinua iltaan asti.

Because sinua is the partitive form of sinä (you), and here you is the object of the verb odottaa (to wait for).

• sinä = you as the subject
• sinua = you as the object, in the partitive

So:

• Sinä odotat. = You are waiting.
• Minä odotan sinua. = I am waiting for you.

In Finnish, odottaa commonly takes its object in the partitive. This is just the normal pattern of the verb.

So you say:

• odotan sinua = I’m waiting for you
• odotamme bussia = we’re waiting for the bus

A useful way to think about it is that waiting is usually seen as an ongoing, not-yet-completed action, and that often goes together with the partitive in Finnish.

Odotan is the 1st person singular present tense of odottaa.

So:

• odotan = I wait / I am waiting
• dictionary form: odottaa = to wait (for)

Finnish present tense often covers both English present and near future, depending on context. So odotan can mean:

• I am waiting
• I wait
• sometimes I will wait

Finnish very often uses the present tense where English might use will.

So Odotan sinua iltaan asti can naturally mean:

• I’m waiting for you until evening
• I’ll wait for you until evening

The exact time sense comes from the context, not from a special future tense form, because Finnish does not have a separate future tense like English does.

Iltaan asti means until evening.

It has two parts:

• iltaan = to/until evening
• asti = up to / until

Together they form a time expression meaning that the waiting continues up to that point in time.

So the whole sentence means that the speaker will wait for that person until evening arrives.

Iltaan is the illative singular of ilta (evening).

The illative often means into, to, or in time expressions up to a certain point. In this sentence, it marks the endpoint of the waiting.

So:

• ilta = evening
• iltaan asti = until evening

This is a very common way to express a time limit in Finnish.

Because illalla means in the evening / during the evening, not until evening.

Compare:

• Odotan sinua iltaan asti. = I’ll wait for you until evening.
• Odotan sinua illalla. = I’ll wait for you in the evening.

So the difference is:

• iltaan asti = endpoint in time
• illalla = time when something happens

Usually yes, if you want to clearly say until evening.

• iltaan asti = until evening
• iltaan saakka = also until evening

Without asti or saakka, odotan sinua iltaan would sound incomplete or unnatural to most learners’ ears in this meaning.

So asti helps make the time-limit meaning clear.

Yes. Asti and saakka are both used to mean until / up to.

So these are both natural:

• Odotan sinua iltaan asti.
• Odotan sinua iltaan saakka.

They are very close in meaning. In many situations, the choice is just a matter of style or preference.

Yes. Finnish word order is more flexible than English word order, although the basic order here is very natural.

Default order:

• Odotan sinua iltaan asti.

But you could change the order for emphasis, for example:

• Sinua odotan iltaan asti. = emphasizes you
• Iltaan asti odotan sinua. = emphasizes until evening

The basic meaning stays the same, but the focus changes.

Yes, if the context already makes it clear who is being waited for.

• Odotan iltaan asti. = I’ll wait until evening.

Adding sinua makes the object explicit:

• Odotan sinua iltaan asti. = I’ll wait for you until evening.

So the full sentence is more specific.

The verb odottaa can mean both wait for and expect, depending on context.

For example:

• Odotan sinua. usually means I’m waiting for you
• Odotan hyvää tulosta. can mean I expect a good result

In Odotan sinua iltaan asti, the time expression iltaan asti strongly supports the meaning wait for you until evening, not expect you until evening.