An easy way to see it without difficult grammar explanations, is trying to invert the sentence and see if it works and means the same that way: Mi manĝas pano = I eat bread... right? but how about Pano manĝas mi? who is eating who here? here's where the helpful -n comes to rescue. It doesn't matter how you structure the sentence. The eaten one will be then with the -n: Mi manĝas panon = I eat (a) bread Min manĝas pano = Me, a bread eats. (obviously this is too complicated to use) Panon manĝas mi = Bread I eat ( like Yoda ) Pano manĝas min = A bread is eating me! I am horrible at grammar but I believe you just need to understand why some things are used to start using them. Hope I helped!
I think it's because "pano" is the nominative or normal form that you would find in a dictionary for example. When you say you or someone else is eating bread you use "panon" e.g. Mi mangas panon" (sorry I don't have the accent) because it becomes an accusative form and so you attach and n on the end. I hope that makes sense.
It's improper English. If you're discussing food in a quantitative amount, then you must have a. If it's in an abstract amount, you must not use a. Examples of quantitative amounts of food would be things like "a bar of chocolate," "a slice of pizza," and "a juice box." (You can take out the of and put the food behind the amount; 'I eat a chocolate bar.') Examples of non quantitative amounts of food would be just the noun - "I eat chocolate," "I eat pizza," and "I drink juice." Hope i helped! :)
Note that some foods imply a quantitative amount and don't need an amount. You can eat an apple, a pizza (which would be the whole thing, rather than a slice), or a cow (again, this would be the whole cow). At least, you can grammatically, your milage may vary on being able to eat that much food.
The "do" is unneeded, so all it does is add emphasis, implying someone denied that you eat bread in the café. Basically, here are two situations with and without do in their correct usages:
Without do: "Do you eat bread in the café?" "Yeah, we eat bread in the café every day"
With do: "You do not eat bread in the café." "We do eat bread in the café, actually."
It has to be in the accusative, panon. When you have a subject acting on the direct object via a transitive verb, you indicate that object with the accusative -n ending. It allows you to rearrange the word order.
Put simply English uses Subject-Verb-Object word order, but since you can have different word orders in Esperanto, you need to id them. They use -n on the Direct Object (the word the verb acts on) to do this.