Learn Polish: Moi przyjaciele mieszkają w Polsce.

In Polish plurals, we divide things into two groups: "masculine personal" (groups with at least one man) and "everything else" (women, animals, objects). Because przyjaciele (friends) implies a group with at least one male, we must use the masculine personal form Moi. If it were a group of only female friends (przyjaciółki), you would use Moje.

Is przyjaciele a regular plural form?

No, it is an irregular plural of the noun przyjaciel (male or general friend). You just have to memorize that one friend is przyjaciel and multiple friends are przyjaciele. Many masculine nouns ending in a soft consonant have slightly unpredictable plurals.

Why does Polska change to Polsce at the end of the sentence?

The preposition w (in) requires the Locative case when you are talking about a static location. For feminine nouns ending in -ka like Polska, the Locative ending changes the -ka to -ce.

Could I use the verb żyją (they live) instead of mieszkają here?