Is Y'all Singular or Plural? It's Both.

Also, why do we say "hold on tight" — shouldn't it be "tightly"?

Matthew Jedras asks:

I’m learning Russian now, and one of things that has struck me is how, when you would use an adjective in English to say “Wonderful!” or “Terrible!,” in Russian you use an adverb — “Otlichno!” means “Wonderfully!” and “Uzhasno!” means “Terribly!”

That got me thinking about English. A lot of times I hear “Hold on tight” or “I pack light.” These sound right to me, but then I think that it should really be “Hold on tightly” and “I pack lightly.” Am I wrong to draw this throughline between Russian and English?