I may be wrong as I am not native in English, but: "They buy two bananas." is a correct one here, while "they are buying..." is not. However, "The girl is buying mämmi, cream, and sugar." is correct for "tytto ostaa mammia, kermaa ja sokeria", while "The girl buys..." is not. Although I "feel" the difference between two situations, I want to ask: is it absolutely clear for native speakers (or just people who now English better than I do) that in each of these two cases you have indeed only one specific way of translation? And this is not unique: in some phrases I do not feel rock-solid certainty regarding "eats" vs "is eating" and so on.
I think 'He ostavat kaksi banaania' can be translated as both "They buy two bananas" and "They are buying two bananas".
In https://forum.duolingo.com/comment/42942450, someone wrote that 'Minä ostan kaksi kakkupalaa' could be translated as either "I am buying two pieces of cake" or "I buy two pieces of cake", since using the partitive object, as in'Minä ostan kahta kakkupalaa', just isn't done for this verb.
It is because it is the object after a numeral greater than one. There are many cases and probably each has many uses, unfortunately. : )
He ostavat yhden banaanin - they buy one banana. He ostavat viisi banaania - they buy five bananas. He rakentavat talon - they build a house. He rakentavat kahdeksan taloa - they build eight houses.