"They see sixteen insects."
Translation:הן רואות שישה עשר חרקים.
So when is dropping י optional and when is it not? Apparently both options are required here but sometimes when I omit י I get marked wrong too.
Any explanation of why one correct example writes ששה and the other writes שישה ? (both are enumerating חרקים)
The rules for writing without niqqud say that you should write שישה. These rules are subtle, though, and not very well known by native speakers. There are cases where omitting the mater lectionis would look very strange, and other cases where it would look quite normal, and I'm not sure it's easy to see the rhyme and reason for each case. FWIW to my eyes writing ששה looks quite natural, and apparently also the authors/editors of this course.
I think this word for insect comes from the Hebrew verb to grind teeth, חרק. It's not a biblical word. The ערב in שמות (Exodus) are unidentified swarming bugs, which usually gets translated as flies in English, but flies is זבוב. Here's a link to a short video of a person talking about modern Hebrew insect vocabulary and she describes how big they can be in Israel: https://www.youtube.com/watch?v=DbkeeZZnTgk