Learn Finnish: Kuuletko sinä?

In Finnish, you turn a verb into a yes/no question by adding the ending -ko (or -kö, depending on vowel harmony). Since kuulet means you hear, adding -ko makes it Do you hear?.

What is the dictionary form of the verb in kuuletko?

The dictionary form is kuulla (to hear). It is a Type 3 verb, which means you find the stem by dropping the -la and adding an -e- before the personal endings. So the stem is kuule-, and adding the -t for the second person gives us kuulet.

This lesson is about partitive verbs. Where is the partitive object here?

You are right that kuulla is a partitive verb! When you hear something, that object takes the partitive case, for example Kuuletko musiikkia? (Do you hear music?). However, this sentence has no object at all. It is just asking if you can hear in general.

Do I always have to say the word sinä in a question like this?

No, it is optional. Because the verb kuuletko already has the -t ending, it is clear that you are talking to one person. You could just say Kuuletko? and it means the exact same thing. Adding sinä just puts a little more emphasis on the person.