This translation of the "-na-" tense contradicts an earlier test item. here -na- is translated as present continuous ("is" doing the verb), elsewhere it is translated as indefinite ("does" the verb). For example, it is the difference between he buys and he is buying, but each time the -na- is used. Based on Simplied Swahili, -na- tense should be used for, e.g., is buying, and the -a- tense for, e.g., buys.
this is inconsistent so far and should be updated here.