Learn Portuguese (Portugal): Eu moro em Lisboa.

moro is the "I" (eu) form of the regular verb morar (to live). To make this form in the present tense, you drop the -ar from the end of the word and add -o.

Can I leave out the word eu and just say moro em Lisboa?

Yes! In Portuguese, the verb ending usually tells you who is doing the action. Since moro can only mean "I live", the word eu is optional and often left out in everyday speech.

I've seen words like no and na for "in". Why does this sentence just use em?

The preposition em means "in", "on", or "at". While countries and some cities require you to combine em with a word for "the" (creating no or na), most cities—including Lisboa—don't use "the". So, you just use the plain word em.

I learned the verb viver means "to live". Why use morar here?