Learn Finnish: Mekin harrastamme urheilua, kun meillä on aikaa.

The suffix -kin attaches directly to the specific word it emphasizes. By adding it to me (we), the sentence means "We also do sports" (just like you or someone else does). If you attached it to the verb instead (harrastammekin), it would mean "We also do sports" (in addition to just watching them, for example).

Why is the word for sports in the partitive form, urheilua?

The verb harrastaa (to do as a hobby or regular activity) is a "partitive verb". It always requires the activity you are doing to be in the partitive case, because a hobby is seen as an ongoing, continuous action rather than something with a clear start and finish.

Why is the verb on used in meillä on aikaa instead of olemme (we are)?

As a quick refresher, Finnish doesn't have a direct verb for "to have". Instead, you use the pronoun in the -llä/-lla form (meillä = on/at us) followed by the 3rd-person singular verb on (is). It literally translates to "at us is time".

Why is aika (time) also in the partitive case (aikaa)?

In "have" structures like meillä on, abstract or uncountable things (like time, courage, or coffee) must be put in the partitive case. It implies we have "some" time.