Learn Finnish: Minä rakastan sinua.

The verb rakastaa (to love) is what we call a partitive verb. This means it always forces the object of the sentence (the person or thing being loved) into the partitive case. So, sinä changes to sinua.

Is sinua formed by adding a regular partitive ending to sinä?

Not exactly. Personal pronouns have their own specific partitive forms that you just have to memorize. For example, minä (I) becomes minua, and sinä (you) becomes sinua.

Can I just say Rakastan sinua without the minä?

Yes, absolutely! Because the -n ending on the verb rakastan already makes it clear that "I" am the one doing the loving, it is very common and completely natural to drop the minä.

Is there a rule for guessing which verbs act like rakastaa and need a partitive object?

A great rule of thumb is that verbs expressing emotions or feelings usually take the partitive case. So verbs for loving, hating, fearing, or admiring someone will almost always require a partitive object.