So why is "bekalut", which ends with a hey spelled thusly, and "b'kushi", which also ends in a hey(and doesn't seem to contain any special letters) spelled this way? Why isn't it "bekashut"?
Neither word has a hey (ה), and they don't end with the same letter - they start with the same two letters, "..בק". (bet and kof)
Forgive me, I was thinking "Kalah" for some reason. "Kasheh" has a a hey at the end, does it not?
Oh, sorry - I didn't understand you then, I thought you were talking about the words בקלות and בקושי..
You're right - קל is easy and קשה is difficult, while קלות = ease and קושי = difficulty, but I'm not sure if there's a general rule here.
Well, I think the group ['XoXi] consists of only a few nouns whose biblical pausal form has been generalised, like יֹ֫פִי, חֹ֫לִי, קֹ֫שִׁי, which were originally יֳפִי, חֳלִי, קְשִׁי, all of them roots with a weak third radical.