Learn Finnish: Veljeni pelaisi lottoa, jos hänellä olisi rahaa. - My brother would play the lottery if he had money.

Pelata is a type 4 verb. To find the stem, you drop the final -ta and add a matching vowel, giving the stem pelaa-. To make it conditional ("would play"), we add the conditional marker -isi-. The double aa drops one vowel to make room for the i, so pelaa + isi becomes pelaisi. Because it is "he would play" (3rd person singular), there is no extra personal ending added.

How do you form "would have" in Finnish, as seen in hänellä olisi?

Remember that Finnish doesn't have a direct verb for "to have". Instead, it uses the structure "at someone is". For "he has", you say hänellä on (at him is). To change this to "he would have", you just change the verb olla (to be) into the conditional mood. The 3rd person singular conditional of olla is olisi, so the phrase becomes hänellä olisi.

Why are both lottoa and rahaa in the partitive case (ending in -a)?

They are both in the partitive case, but for different reasons! Pelata (to play) is a verb that always requires its object to be in the partitive, which is why we use lottoa. Rahaa, on the other hand, is in the partitive because it is an uncountable word (a mass noun) used in a "to have" sentence—he would have an unspecified amount of money.