Learn Finnish: Sinä olet viheltävinäsi, mutta näpräät vain näppäimistöä. - You are pretending to whistle, but you just fiddle with the keyboard.

It uses the quasi-construction (kvasirakenne). You take the present active participle of viheltää (which is viheltävä), change it to the required plural form viheltävinä-, and attach the possessive suffix that matches the subject. Since the subject is sinä, you add -si, resulting in viheltävinäsi.

Does the possessive suffix on viheltävinäsi always have to match the subject?

Yes. The quasi-construction always requires the possessive suffix to agree with the person doing the pretending. For example, 'I pretend' would be olen viheltävinäni, and 'he/she pretends' is on viheltävinään.

Why is the word for keyboard, näppäimistö, in the partitive case (näppäimistöä)?

The verb näprätä (to fiddle with, to tinker) is a partitive verb. Fiddling with something is an ongoing, continuous action without a clear endpoint, so its object must always be in the partitive case.

What verb type does näpräät come from, and how is it conjugated here?

It comes from the verb näprätä, which is a Verb Type 4 word. To conjugate it for sinä, you drop the -tä infinitive ending to get the stem näprä-, and then add the personal ending -ät to get näpräät.