Hi,
When you click a sentence mastered, having to mouse to the mastered confirmation button takes your hands off the keys and is difficult with people with disabilities or repetitive stress injuries, as opposed to having the enter key confirm. Please consider streamliniing and reducing clicks by allowing the enter key to confirm.
Additionally, having a shortcut just for the mastered sentences in general is a great idea as well. So amend, my suggestion: 1) create a keyboard shortcut for master this sentence (it’s really so out of the flow, I usually skip it) and 2) allow the enter key to confirm. Thanks!