Learn Finnish: Kävele varovasti.

The verb is kävellä (to walk), which is a Type 3 verb. To make a command for one person (the 2nd person singular), you find the "I" form of the verb (minä kävelen) and just drop the -n. That leaves you with the command: kävele.

Why is there only one l in kävele, but two in the dictionary form kävellä?

This happens because of how Type 3 verbs work. The infinitive (dictionary form) kävellä has a double ll, but the everyday personal stem uses a single l (like in kävelen or kävelet). Because commands are built from that "I" form rather than the dictionary form, the command is kävele.

Is the word sinä (you) missing here?

It is omitted on purpose! In Finnish, when you give a command to one person, you just use the command form of the verb (kävele). You don't need to say sinä kävele, just like in English we say "Walk carefully" instead of "You walk carefully."

How does varovasti mean "carefully"?

The word is built from the adjective varovainen, which means "careful". By adding the ending -sti, it turns into an adverb (a word that describes an action). The -sti ending in Finnish works exactly like the "-ly" ending in English!