Hi:

Principle:  If you want to know anything, you must not try to know everything - at least not initially.  So, focus on the essential like a laser; else, especially if your systems are complex, you will wide up with a bunch of details, but only a disjointed understanding of the essential.  Gathering the essential is hard work, you really have to be hard headed about maintaining a proper focus.

Given the above, what are the questions to ask?    Think of the primary purpose of a BA:  To understand the essential (unchanging, technology independent)  business processes/tasks to be accomplished, and - especially - the interrelationships between them.

Ask subject matter experts what are  input/process/outputs.   A process is defined by its inputs and outputs.   Add rigror to your questioning by creating and walking thorugh via integrated input/process/output  diagrams (data flow diagrams).

And your questions need to be as high level as possible.  Save the details till the appropriate time.

Tony