Learn Finnish: Päätin ruveta harrastamaan uutta kamppailulajia. - I decided to start practicing a new martial art.

The verb ruveta (to start or to get down to) always requires the following verb to be in the 3rd infinitive illative form, which ends in -maan or -mään. If you wanted to use alkaa (to begin) instead, it would take the basic dictionary form: alkaa harrastaa.

Can you string three verbs together like päätin ruveta harrastamaan?

Yes! In a Finnish verb chain, each verb dictates the form of the verb that comes immediately after it. Here, päätin (I decided) requires the next verb to be in its basic infinitive form (ruveta). Then, as we just learned, ruveta forces the third verb into the -maan form (harrastamaan).

Is there a difference in meaning between ruveta and alkaa here?

They mean essentially the same thing, but ruveta is slightly more colloquial and often carries a nuance of "getting down to work" or taking the initiative to do something. The most important difference for you as a learner is their grammar rules (the forms they force the next verb to take).

Why are the words for "a new martial art" (uutta kamppailulajia) in the partitive case?