Learn Swedish: Titta! Katten sover. - Look! The cat is sleeping.

Swedish doesn't have a direct equivalent to the English "-ing" form (the present continuous tense) for ongoing actions. Whether a cat "sleeps" every day or "is sleeping" right this second, you just use the standard present tense: sover.

Why is it Titta! and not Tittar!?

Because this is a command (the imperative form). When telling someone to do something, you don't use the present tense -r ending. For verbs like titta, the command form is the exact same as the basic dictionary form (att titta).

Where is the word for "the" in Katten sover?

In Swedish, "the" isn't usually a separate word. Instead, it gets attached to the end of the noun. Because a cat is en katt (an en-word), you add -en to the end to make it definite: katten (the cat).