"This is all very sudden."
Translation:Das ist alles sehr plötzlich.
"alle" would be plural, but "das" is singular, so it's followed by the singular "alles". In both English and German, you treat "all this" (as well as "everything / alles") as a singular thing (you don't say "All this are very sudden" or "On this old TV, everything are grey").
Two examples for plural: "Can you find me a different knife? All of these are blunt." = "Die (or: diese hier) sind alle stumpf." (...and, for the record: "These are totally blunt" = "Diese hier sind ganz/völlig/total stumpf"); "Let's pluck some strawberries! ...Oh no, they're all still green." = "Die sind alle noch grün."
"das ist alles sehr jah" doesnt work either. What the actual (substituted with a long text instead be cause i fear i may be banned) is wrong with duolingo showing various translations under a word and then only accepting one. And if anyone tries to argue saying something along the lines of 'well you see, x doesnt fit the context here' why does duolingo not give any indication of that? Phew, long text. I guess i wanted to vent.