"I eat bread."
をis the object marker, while は is the topic marking particle. A good way to think of は is that it is used to bring context into the sentence, and to bring new information as well. Take for example, you are having a conversation about the bread, and how you are eating it. If you have already brought up the bread, you don't need to stress the context. This is like " watashi wa" at the beginning of a sentence. It is already clear that you are talking about yourself, but if you want to make it clear without stressing it, you would use は. Now, if you weren't talking about how you're eating bread, and you want to bring it up in a sentence, and stress it, with the を. So, [パンは食べます] is technically not incorrect, but in this instance, I believe, it is more correct. Sorry if it was confusing, but I hope it helps!</pre>
This is the me from the future, and I think I might have a better, clearer answer.
Since は is the topic particle, it is usually used to bring up a new topic into conversation. An example of how to translate [パンは食べます] would then be "As for the bread, (I'm) eating it.". And, since を is the object marker, you can think of it as being used to show that the word before it is the direct object of the verb. So in the case of パンを食べます, it would translate into something like, "(I'm) eating bread.", yet it places more emphasis on the action, rather than the bread.<pre>
What I'm trying to say is that you would reply using は if you wanted to bring it to the other person/s attention that that is what you're eating, while you would use を to convey that you are eating. Both are correct, but have different contexts behind them. I hope that this is easier to understand than what I put before!</pre>
を is the direct object particle, so if you use that to note bread as the direct object, then it directly translates to "I eat bread."
は is the topic marker. It is not necessarily the subject of the sentence. If someone were to say パンは食べます, it would indicate bread as a general topic. It's like saying "Bread? Yeah, I eat that."
Wa is used to mark the topic of the sentence (we all know that) but also when talking about when referring about something negatively. As in "i do NOT eat bread" or "i am not a student" "O" can be used for temporary objects such as food. So when you talk about how you do eat this food you'd use it. If you do not eat it you'd use "wa" I imagine theyre may be exceptions but i dont know of them. Sorry for not using the correct characters. I'm on my phone
I've heard from a different thread that since wa is the topic marker, it puts emphasis on the thing that comes after. So if you say that you don't do something the focus is on the fact that you don't do it, hence why you use wa in negative sentences. And from what I understood, wa in this sentence would mean that you put emphasis on the fact that you eat it rather than what you eat. I imagine the wa to be like "I EAT the bread! What else would you do with it"
I'm guessing you left out the 私は, with 私 (watashi) being I/me, and は being the sentence topic marker. パンを食べます is still a grammatically correct way of saying "I eat bread." You would say it like this when it could be understood from the context that the speaker is the one who eats bread.
This answer accepts the short/dictionary form "パンを食べる," which i think is interesting. It's been years since I studied, but if I remember correctly the short and long forms are interchangeable. 食べる (tabeRU) becomes 食べます (tabeMASU). Drop る add ます.
Generally I believe る is the actual dictionary form "eat", while ます is a future tense, "will do".