If you talk by heart, you should say: わたしは　おちゃが　すき　じゃない if by mind, you can say わたしは　おちゃを　すき　ではない、 わたしは　おちゃを　好まない わたしは　おちゃを　愛好しない Generally 好きだ、嫌いだ、～したい kind of verb should have "が“ before itself. OK: お茶が好きです。 OK: お茶が好きではない。 OK: お茶は好きではない。 However, nowadays "を“ is often used. OK: お茶を好きです。 OK: お茶を好きではない。 You can say that and nobody misunderstand you. Why? It is because modern Japanese is influenced by English and other Western languages. I think you use ”が” when you talk by heart and you use ”を“ when you talk by mind. 私は真理を知りたい　 　　　vs　 本当のことが知りたい あー、水が飲みたい！ あー、水飲みたい！ メシ食いたい ピアノ弾きたい! マンガ読みたい! - A student in a music school says: 「今度の発表会では、バッハのピアノ協奏曲3番【を】弾きたいと思います」 　　ｖｓ 「オレはなあ、どうしてもピアノ協奏曲3番【が】弾きたいんだ。 それが完璧に弾ければ、もう死んでもいいとさえ思ってるんだ」 For details, please refer to: https://www.italki.com/question/425077 >One of the above sentences ended in janai desu, and one ended in janai. Is it more correct > to add desu? Is it informal to leave desu off? I recommend you basically use DESU/MASU. It is usual Japanese.

For na-adjectives (which suki is), the more casual conjugation of the negative is janai. The more polite way is deha(wa) nai. Ga is used after ocha (noun) because it is showing the subject of the "not" like. Nouns can also be conjugated, which is why there isn't anything after gakusei, just the negative conjugation. Janai after the noun, just like with the na-adjective, conjugates the same. As Ueda said, they are grammatically correct, but it is just a level of politeness overall.

My pleasure.

Thank you all for your help!

Both are grammatically correct. It would be more polite if you put "です" and "ます" at the end of sentences, though.