Learn Finnish: Jos sataa, muutamme suunnitelman. - If it rains, we will change the plan.

English uses a "dummy" subject for weather, like "it" in "it rains". Finnish doesn't do this. You simply use the verb in the third-person singular form all by itself: sataa (rains).

The translation says "we will change", but isn't muutamme in the present tense?

Yes! Finnish does not have a dedicated future tense. You use the present tense for things happening in the future. The condition jos sataa (if it rains) gives enough context to show this is a future plan.

Why is the word suunnitelma written with an -n at the end (suunnitelman)?

It is the direct object of the sentence. Because we are changing the entire plan, it takes the object case (often called the accusative), which usually ends in -n and looks identical to the genitive case.

Could I say jos sataa, me muutamme suunnitelman by adding the word for "we"?

Absolutely. Because the verb muutamme ends in -mme, it already clearly means "we". Finnish speakers often drop the pronouns minä (I) and me (we) for this reason, but keeping me in the sentence is completely correct.