Learn Finnish: Minä tunnen sinut hyvin.

Sinut is the accusative form of "you". We use it here because "you" are the direct object of the sentence—the person being known. In Finnish, personal pronouns have a special -t ending (like minut, sinut, hänet) when they are the total object of an action.

I thought the verb for "to know" was tietää. Why are we using tunnen here?

Finnish uses two different verbs for "to know". Tietää is for knowing facts or information (like knowing an answer or a password). Tuntea (which becomes tunnen for "I") is used for knowing people or being familiar with places. Since you know a person in this sentence, you must use tuntea.

Can I just say "Tunnen sinut hyvin" and leave out minä?

Yes, absolutely! The -n ending on the verb tunnen already makes it perfectly clear that "I" am the one doing the knowing. In everyday Finnish, it is very common to drop the pronouns minä (I) and sinä (you).

Why is it hyvin and not hyvä?