Learn Finnish: Voitko sinä tulla?

The suffix -ko (or -kö) is added to the first word of a sentence to turn a statement into a yes/no question. Since voit means "you can", adding -ko makes it voitko, meaning "can you?".

Do I have to include the word sinä (you)?

No, it's completely optional! The verb voitko already has the "you" ending (-t), so voitko tulla? makes perfect sense on its own. Including sinä just puts a little more emphasis on the person you are asking.

Why is the second verb tulla in its basic dictionary form?

When you use an auxiliary (helper) verb like voida (to be able to / can), only the first verb gets conjugated to match the subject. The second verb, tulla (to come), stays in its basic infinitive form. It works just like English: we say "can you come", not "can you comes".

Is voida or tulla a Verb Type 4, since that's what this lesson is about?

Actually, neither of them are! Voida (to be able to) is a Verb Type 2 (verbs ending in -da/-dä), and tulla (to come) is a Verb Type 3. Even though you are learning Type 4 verbs right now, you'll still see common helper verbs like voida popping up alongside them constantly.