Can you play one player on new super Mario bros Wii?

Answer 1

No, not in Brawls.

But you can do so in Training Mode, good way to test out a few moves and train on combos, especially since they show damage and how many hits you've done.

example: Try to figure out your main characters most damaging attack, this could be useful when your opponent is Stunned, but be aware that the more hits a Specific attack does, the weaker it becomes: Say, 17% - 11% eventually.

I know allot of things you could test but be a little creative and it'll pay off.

PS: The Weakening of an attack wears off when you Respawn,
But this effect doesn't apply to some Specials - Test it.

Answer 2

Yes of course, but its more fun with 2,3, or 4 people.