Hey and welcome on the community forums! 
You almost got it!
The corrected version:
貴重品は車に残さないで下さい。
I have linked the grammar point that is used to say “please don’t (do something)”.
I hope it helps,
Cheers!
PS
If you want to forbid doing something, you can also use “must not” construction - てはいけません。
But this is much stronger and is mostly used on things like signs that forbid entering the construction site, etc.
入ってはいけません。
Entry is forbidden/Do not enter/You must not enter, etc.
Thanks 
I think I’ve heard that before…in Naruto, perhaps. I’ve definitely heard 「行かないで」 in the ending theme, Long Kiss Goodbye - I looked up the lyrics to be sure. I’m not too far off that grammar point now…although I suppose I can do it whenever.
I suppose forbidding people from leaving things in their cars would be a bit heavy-handed! Would that be suitable for, say, asking people not to touch works of art in a museum?
As for another sentence, how about:
He works at the library.
Sounds good.
If you are at the position of authority it should work.
Those three might be a bit better:
彼は図書館で仕事をしています。
彼は図書館に勤めています。(There is a light novel and anime series called はたらく魔王さま, working demon king)
彼は図書館で働いています。
Hope it helps,
Cheers
So I began 5 days ago with Japanese and just made my first sentence, but I have some questions about it. If someone could help, it would be appreciated.
ブルフィンは犬です
I’m trying to understand the difference between は and が. I think I used it in this sentence right, but I’m not sure.
Also do you always need to put です at the end of the sentence for ‘is’?
お客様の本日のご計画を伺ってもよろしいでしょうか？
お客様の本日のご計画を伺ってもよろしいでしょうか？
O honored guest, would it be quite alright if I was to inquire as to your keikaku for this fine day?
(TL Note: keikaku means plan)
I put off the keigo grammar points for like 3 months but now I’m finely getting to it and it’s kinda fun. Sounds so fancy. wwww
Hey and welcome on the community forums! 
I will write more about it in the future, but for time being, those two answers should be helpful.
(those are pretty long though!)
You don’t have to! It all depends on how polite do you want to be.
So if you are speaking with some person you don’t know, someone older, someone you are not close with or higher in the hierarchy (like your boss) you should use です.
When talking with friends, people from the same class, or when you are higher in the hierarchy you don’t have to use です.
(very simplified explanation)
So:
ブルフィンは犬です (talking with an unknown person, someone older, etc)
ブルフィンは犬だ (talking with friends, people from class, family, etc)
ブルフィンは犬 (talking with friends, people from class, family, etc)
ブルフィンは犬よ (talking with friends, people from class, family, etc but more feminine)
are all correct depending on the situation.
By the way, remember that directly after いadjectives you can use です but must not use だ. This is a very common mistake, even Japanese make.
この車は少し狭い。 (correct!)
この車は少し狭いです。(correct!)
この車は少し狭いだ。(wrong!)
です, in this case, doesn’t work as “to be”, but simply marks that you are using polite speech. The “to be” is included in いadjective itself.
I hope it helps!
Cheers!
It is correct!
In some context the version with が would be also correct! Read the post above for more details 
How is this sentence?
鉄が本来地から来る。
I wasn’t sure about placing 本来, which was the word I was aiming to use.
Wiktionary also mentioned the word 元来 which I didn’t know, but seems like maybe it’s more about natural things than 本来 is. Is there a distinction there?
I was also torn between past and present tense. Present tense seems like it makes the sentence more general somehow, which is what I wanted.
Thanks 
私は本が好き。I like books.
Is that right?
Hi and welcome!
Yes that’s fine! You can also add だ or です if you want, depending on how polite you want your sentence to be.
いくら日本語を勉強しても、私は全然話ことができない。
No matter how much I study Japanese, I can’t speak at all.
This should be 話すこと. Other than that, fine 
And I was sat there going “Do I add it，or do I not?” 
Sod’s law!
Hello!
Here’s my first sentence and first post in the community:
この会社が興味がありますからお申し込みいたします。
I’m not really sure how to use kanji to show furigana yet.
Translation
I will apply for this company because I am interested in it.
I’m not confident enough with 謙譲語 (the humble mumbo jumbo) to comment on that, but I can at least tell you that 興味 uses the に particle, so the beginning should be「会社に興味が…」
今は臨時休校中だから同僚と一緒に体育館でバドミントンをやってきた。俺は痩せないといけないから超疲れて汗はめっちゃ出ちゃった。これからもっと運動すればいいよな。
明日、大切な人は私の仕事に来ます。
私の部屋はきれいじゃありません。今日少し片付けりました。
HI! Welcome to the community!
Gramatically perfect, but I don’t feel like in Japanese, 仕事 is treated as a place as much as it is in English, so I would say 会社（かいしゃ） or 職場（しょくば） to sound more natural. Would be good if someone could confirm this for me.
You don’t need り here. You can either say 片付けました or 片付けをしました (the を here is optional.)
Keep your sentences coming! It’s a shame that this thread has been really quiet recently.