Pressing TAB when translating single words
This is a very strange (and frustrating for me behaviour). Most of the time pressing tab switches the focus from the text input to the check button. Except when translating single words you need like 6 tab presses to get there. And this is not the only one, there are also other specific pages that have this problem. I think pressing TAB should alway switch immediately from input to check.
I believe that
tab functionality is something provided by your browser, rather than Duolingo. On Firefox IIRC once you've input an answer just hitting enter should submit it. So hit a number key corresponding to the answer from a collection of options, tab to enter any text required, and then hit enter. It doesn't work with Midori unfortunately which is what I'm presently using.