Learn Polish: Rok temu mieszkałem w Polsce. - A year ago I lived in Poland.

The word temu means "ago", and just like in English, it always comes after the amount of time in Polish. For example, you can also say dwa dni temu (two days ago) or tydzień temu (a week ago).

How would a woman say "I lived" in this sentence?

A female speaker would say mieszkałam. In the Polish past tense, verbs show the gender of the speaker. The ending -łem is used by a man, and -łam is used by a woman.

Why do we use the imperfective verb mieszkałem here?

Living somewhere is considered an ongoing state or a background situation, rather than a single, quickly completed action. In Polish, when you describe past states, continuous actions, or habits, you use imperfective verbs.

Why does the word for Poland change to Polsce here?

The preposition w (meaning "in" or "inside") requires the Locative case when you are talking about a static location. For the feminine noun Polska, the Locative case changes the final -a to -e, which forces the preceding k to soften into a c, giving us w Polsce.