[Mesorah] az + imperfect

[Sholom Simon via Mesorah]

Recently I was discussing with some folks on this list (or what it 
_on_ this list?  My memory ain't what it used to be) about the phrase 
"Az yashir", and somebody pointed me to an on-line grammar/diduk 
translation that pointed out that "Az" and "terem" are followed by 
the imperfect tense.

But there are a number of times in this parsha (e.g., 4:26, 4:10) 
when "az" is not followed by the imperfect.

Thoughts?

-- Sholom