Does the preffix yl- imply imperative?
It does not so much imply it as explicitly state it. yI- is the most common imperative prefix.
Can you be more specific about your question, please?
Why is it yI and not bI?
Because it is a command to speak and not just a statement saying, "you do speak". This unit is intended to give you some useful phrases to memorize and you will eventually learn how all the grammar works as you go through the rest of the course.