Learn Finnish: Me lakkaamme pelaamasta puistossa.

The verb lakata (to stop or cease) requires the verb that follows it to be in the third infinitive elative form (-masta or -mästä). In Finnish logic, you don't just "stop doing" something; you "stop from doing" it.

How do you build the form pelaamasta from the dictionary verb pelata?

First, find the strong stem of the type 4 verb pelata, which is pelaa-. Next, add the third infinitive marker -ma-. Finally, add the elative ending -sta (which is the ending that normally means "out of" or "from").

Why does the verb lakata become lakkaamme with two 'k's?

Lakata is a type 4 verb, which means it undergoes "reverse" consonant gradation. The dictionary form has the weak grade (one k), but when you conjugate it to make the stem, it changes to the strong grade (double kk), giving us the stem lakkaa-. Then you just add the "we" ending -mme.

Since the sentence involves stopping, why is the park just puistossa (in the park) and not in a "from" form?