The infix might change depending on what the object was, right? For example if we were taking about an onion, it would read "usikipike."
Correct! All variants should be accepted. :)
Just remember that this is only one of the variants. It could be
usikipike (kiazi) etc
Is this a negative imperative? So in this case we have both subject and object infixes?
Yes, u- is the subject prefix and -si- is the negative and -i- is the object infix.
This particular course is really badly done....no context for many of the object markers