This is explained in the tips and notes for this unit, which you can find behind the lightbulb icon on the website version after you've chosen a unit.
The tips and notes for this unit are at https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes.
Have a look at the "Verbs" section.
Please always read the tips and notes for a new unit before starting the unit - they contain valuable background information on the language and the culture, but especially on how the language works.
So....I'm a little confused here....so in some instances bI is a prefix meaning no object you....but in others Da does.....so in which instance is each their definitions....