Learn Finnish: Me lähdemme kiipeämään.

After verbs of motion like lähteä (to leave, to go), Finnish uses the third infinitive in the illative case. This adds -maan or -mään to the verb's strong stem. It acts like saying you are moving physically into an action.

What is the dictionary form of kiipeämään?

The basic verb is kiivetä (to climb). It is a type 4 verb, which means its stem changes quite a bit before you add endings, including a consonant change from "v" to "p". The strong stem used for this -mään ending is kiipeä-.

Why doesn't lähdemme just take the basic dictionary form of the next verb, like in English?

Unlike English where we simply say "leave to climb", Finnish groups verbs differently. Auxiliary verbs like voida (can) or haluta (want) take the basic dictionary form. But verbs of motion act like a direction, so they require the -maan/-mään ending instead.

Does lähdemme always mean "leaving" or can it mean "going"?

Lähteä primarily means "to leave" or "to depart", but in everyday Finnish it is constantly used to express "going out to do something". So Me lähdemme kiipeämään is a very natural way to say "We are going climbing".