Learn Italian: Prendi le pinne e controllale prima di tuffarti! - Take the fins and check them before jumping in!

When the person doing both actions is the same (you are taking the fins, and you are jumping in), Italian uses prima di followed by the infinitive. While English often uses the "-ing" form ("before jumping") or a full phrase ("before you jump"), Italian requires the infinitive here.

How exactly is the word tuffarti formed?

It comes from the reflexive verb tuffarsi (to dive or jump into water). Because it follows prima di, the verb must be in the infinitive form, but you still need to match the reflexive pronoun to the person you are talking to ("you"). You do this by dropping the final -e from the infinitive tuffare and attaching the pronoun -ti.

Why is le attached to the end of controllale?

Le is a direct object pronoun meaning "them", referring back to the feminine plural noun le pinne. When you give an informal command (using the tu form), Italian requires you to attach the pronoun directly to the end of the verb. So, controlla (check) + le (them) becomes controllale.

Both prendi and controlla are commands for "you". Why does one end in "-i" and the other in "-a"?