1 - good point, the best way to achieve this at the moment is to click to unhighlight the first word before clicking the second one. Even after clicking to unhighlight, the word will still remain in your word list, the idea being that it usually doesn't hurt to collect too many words, and the decision on which words to keep, and which to discard shouldn't take up valuable brain cycles while reading. The flashcards will prioritize the high frequency (most useful) words for you and if any extra words you don't want appear at that stage it's quick to delete them.

2 - the way it's meant to work is that if you select "the plane", then extend it to "the plane is", the larger phrase will overwrite the shorter one. It's definitely true that testing the translation feature can result in a lot of garbage appearing that you don't want.

I could add a collect words ON / OFF setting but obviously that would complicate the UI, so I'm resisting it.

Thanks for your feedback. I actually applied to YC with this a couple of years ago but the product was very immature, I'm a solo founder and I didn't get invited for interview. I'm more inclined to bootstrapping rather than taking VC which seems to be the path most (all?) YC companies take. I may re-apply for summer 2016 though since it sounds like an amazing experience.