Use of "wieder"
Why is it correct to say "ich komme wieder" but it's not correct to say "wir essen brot wieder"?
"Ich komme wieder" means "I'll be back" and "wir essen brot wieder" means "We eat bread again" or "We are eating again." It's not the same I guess. I don't understand your question completely.
I don't know, but you just added a noun that wasn't there before. I am positive there is a correct and easy explanation, but I can't think of exactly what it is at the moment. It has to do with word order.
No doubt, the right word sequence is: "Wir essen wieder Brot" . Duo is absolutely right. So in both cases the adverb "wieder" is following the verb. In the second example an object is added. There are some situations (mostly with two adverbs or following subordinated clauses that may allow another word order. But these exceptions that have to be learnt as a piece of vocabulary. For example "immer wieder". Er besuchte immer wieder das Museum. Er besuchte das Museum immer wieder. Both are correct. But to my ears the second sounds even smoother. But be careful. "Er ging sehr oft in das Museum" is right, "er ging in das Museum sehr oft" is wrong, if it is not meant as stressing "sehr oft" extremely. Normally it sounds wrong. So they safe word sequence is subject - verb - adverb - object .