"I eat rice."
ごはんを食べません isn’t wrong, but in most cases it’s more natural to use -は with the negation. It places the focus of what you’re negating on the noun: “I’m not eating rice (but I do eat other things).”
Compare this to “I don’t eat rice (I just cook it),” where the focus would be on the verb (and presumably it would be more natural to use -を here to prevent the noun from being in the focus).
The emphasis when using は would be on the noun, but if you're using を in this sentence it would mean not that you "don't eat rice," but rather that you won't in this situation. ごはんは食べます >>> In general (as for...), rice I eat. ごはんは食べません >>> In general (as for...), rice I don't eat. ごはんを食べます >>> I'm going to eat rice (either in a second or sometime soon) ごはんを食べません >>> I'm not going to eat rice (right now or in the immediate future, even though there's the chance of it later).
There's a stark distinction to be made between the use of は as an emphasis/topic marker and the use of を as an object marker. The way Duolingo has this set up, it seems as though they have no distinction between "in general" and "specific instances."
No, it has nothing to do with the subject. The-ます ending expresses respect towards the listener. Its function is somewhat similar to the t-v distinction that many European languages make in their pronouns: French tu vs. vous (these are where the term "t-v distinction" comes from), Spanish tú vs usted, Italian tu vs. lei, Welsh ti vs chi etc. Only in Japanese you have to put a special ending on the verb, which means you always have to mark politeness, regardless of whether or not the pronoun "you" appears in the sentence (and there are some cultural differences in the definition of "person of respect" of course).
Btw, the informal ending doesn't have to be -る; it's simply the plain verb as you would find it in the dictionary. So for example "I drink tea" in plain speech is: おちゃをのむ。 (のむ being the dictionary form of "to drink". With the -ます ending it becomes のみます).
-です is used after nouns or adjectives. You can think of it as “am/are/is”. -ます is a verb ending. Both indicate polite speech (keigo 敬語, roughly equivalent to using polite “Sie/vous/Lei” in German/French/Italian, but social customs do vary somewhat). The verb ending also incorporates non-past (i.e. present or future) tense.
For the informal equivalent you can use either nothing or -だ for nouns and na-adjectives (no ending for -i adjectives though), and the dictionary form (the one that ends in -u) for verbs:
- noun: 学生です “am/is/are a student/students” --> 学生(だ)
- i-adjective: いいです “am/is/are good” --> いい
- na-adjective: 好きです “am/is/are likable” (= [someone] likes…) --> すき(だ)
- Ichidan (“ru”) verb: 食べます “see(s)” --> 食べる
- Godan (“-u”) verb: 行きます “go(es)” --> 行く
I advise you use the polite forms unless the person you’re talking to is significantly younger. If you use polite form too much you might earn a bit of a chuckle, but that’s much, much better than offending people ;)
Yep, that’s one of the many food puns in DB character names ;) They started with Goku’s name (which happens to be one of the few that is not a pun; it’s simply 孫悟空 [そんごくう], the name of the mythical Monkey King that he is based off of (the Mandarin form is Sūn Wùkōng)), and then changed the last syllable to 飯 [はん], so it sounds like ご飯 “rice” (but they left the first character untouched, so Gohan’s name is spelt 悟飯 instead of ご飯 (or 御飯 if you want to be traditional)) in Kanji.
The -て form is not a sentence-final ending, instead it links the clause to a following one. It is rather unspecified though; often English “and” is a good first guess, but in the right context it can also correspond to “if”. Or it might simply link the main verb to a following auxiliary, so it remains untranslated. In any case, it needs something following.
There is one big exception: -て can also mark the plain (non-polite) imperative:
- これを食べて。 (Eat this.)
This is because in this case it’s actually an abbreviation: There is an implied くれ “do!” (the plain equivalent to polite ください) after it.
For sentence endings (in the affirmative) you use either the -ます ending (when speaking politely), or the dictionary form of the verb (the one ending in -u; in this case 食べる) when not being polite.
It depends on the larger context. If you’re saying this out of the blue, you would probably use -を, but if you’re already talking about the rice, or if you want to imply a contrast with other foodstuffs (“I eat rice [but not pasta]”), you would use -は.
Since there is no context provided in Duolingo, both versions should be required here.