My comments:
1 - it is not clear which words go into "my words" e.g. I mark one, then the one after, but I don't want it as an idiom, I just want two separate words.
2 - this causes some confusion when going to the flash cards I get (imagine it's in a foreigh language) "the plane", "the plane is" "plane is taking off" "plane is taking of the ground" - each in a single "flash card item". I'm sure it's due to me abusing / testing marking words
I don't have UI / UX solutions to both issues sadly, I'm sure you gave it a lot of thought.
Otherwise, this is a great, fully functional website, and I wish you will make a lot of money. I'm tempted to become a paying user. Looking at DuoLingo's recent valuation, I would say that there is a market for this.
Great example of a real bootstrapped SaaS.
Hope you get to see it grow further. Sounds like a valid YC submission if you ask me, worth a shot...