Learn Finnish: Poliisi pelastaa lapsen. - The police officer rescues the child.

Actually, no! Pelastaa (to rescue) is a Type 1 verb. Because it ends in two vowels, it might remind you of Type 4 verbs (like pelata). Here it is simply in the standard 3rd-person singular form (he/she/it rescues), which doubles the final vowel.

What would the verb look like if we used the imperative mood to say "Rescue!"?

To form the singular imperative for a Type 1 verb like pelastaa, you take the "I" form (minä pelastan) and drop the -n. This gives you the command form: Pelasta!

Why is the object lapsen and not the basic dictionary word lapsi?

Because this is a normal indicative sentence where a subject (poliisi) is completing an action on an object, the singular object needs the accusative case. For singular nouns, this looks exactly like the genitive case (ending in -n). The word lapsi changes its stem to lapse- before adding the -n.

If this sentence were an imperative command ("Rescue the child!"), would the object still be lapsen?

No, and this is a crucial rule to remember when practicing imperatives! In a command, the object drops the -n ending and uses the basic nominative form. So, "Rescue the child!" would be Pelasta lapsi!

Why does mean "the police officer" here rather than "the police" in general?