Ein, eine, and einen?
How do I know when to use these words?
Also another thing that trips me up: Der, den, die, and das.
Can someone explain this to me, please?
The Duolingo tips and notes page for the Accusative Case skill should help with that.
On the website version of Duolingo, there's a little light bulb button next to the Start or Practice button for a skill. Click the light bulb for the tips and notes page for the skill.