I do not know javascript right now but I am planning on learning it. My question is do I need to have a good understanding of javascript in order to use jquery? I already know actionscript and php which are quite similar. Should I study javascript before learning jquery?
Probably not, although it wouldn't hurt. Chances are, if you learn jQuery, you'll also wind up learning Javascript along the way.
It depends on what you need to to with jQuery..
of course it helps but most basic operations can be done just calling functions of the framework itself with selectors like:
$('#elementId').hide()
So quick answer: yes, you should but until you don't need complex control flow over jquery you can do simple things just with basic syntax.
EDIT: of course without knowing how to attach functions to elements, or how to execute functions when document is loaded you can't do anything, so these basic things are essential before any kind of jQuery usage.
jQuery helps you to use the more advanced aspects of Javascript without a deep knowledge of it. However, it would be helpful for you to get a general overview before you dive in to jQuery.