Translation:I eat vegetables.
How do I tell when to use を or は ? I know they're both topic markers, and I know that は is a topic marker and を is a direct object marker, but I can't logic through why we say やさいをたべます and やさいはたべません。 If anyone can explain it, that'd be great. Is it because I don't eat vegetables so vegetables are the focus, and I eat vegetables has me as the focus? Would it then be correct to say やさいをたべます to put focus on the vegetables? Like when ordering food or something? I guess my thought process doesn't translate well into English, but would that kinda be like saying "I eat vegetables" to state a preference? Like "I eat vegetables" and not meat? I really don't mean to state a comparison because I know that's said differently, but I'm just trying to convey the intention that I'm thinking of. Is my logic/understanding correct?
I understand your reasoning. Using を it is just a simply statement: As for me, I eat vegetables. The topic would be me/watashi は. Use やさいは when you put an emphasise on the fact that you eat vegetables (as for vegetables...), e.g. in comparison to someone else disliking or not eating them. It all depends on the subtlety of the context.
野菜食べる is a fine sentence, it's just really really casual and should just be used with close friends et cetera. The polite form (id est the one you should use) would be 野菜を食べます like above, with the masu form of 食べる and an を to mark the object.
The opinion of someone with more experience than me (I know nothing) would be helpful to weigh in, though.
I can answer the first part. "I will eat" is future tense - something that is going to happen, but hasn't happened yet. This wants the present tense "I eat vegetables" - a statement of present fact. Something that is true now. (This is also different from present continuous tense, which would be "I am eating vegetables", implying that you are eating vegetables right at this very moment.) Unfortunately, I don't know how to make a verb future tense yet, so I can't answer the second part.
やさいは食べます。as for vegetables, I eat them. やさいを食べます。vegetables is a thing I eat. It's subtle. As a general rule, if the topic of vegetables has already between brought up, then use は. If the topic is about asking what you eat, then use を.
What works for me (I'm a learner so I could be wrong), for wa, the new info is usually what's after the wa. With wo, the new information is usually what's before the wo.