"I eat sushi with a fork."
Duolingo may be teaching some people to say things they may want to keep to themselves. . .
I'm a fan of eating sushi with a spork, personally. Or heck, bring a spatula and go to town.
Forks? Chopsticks? Hands? I don't see how anyone could possibly eat sushi with these.
Me? I personally eat with my mouth.
Particles are still confusing. I said fork o sushi de tabe masu. Thought it would be thus that the fork is the object that is used to eat the subject.
You sushi'd the fork by eating. Or something like that.
Correct would be: fork de(by fork), sushi o tabemasu(eat sushi).
Remember the Japanese language doesn't go left to right. It's usually the word on the right of the particle modifying the left side.
The verb is to eat, so the objet (what you eat) marked with を is the sushi. You are not eating the chopsticks.