"Mr. John is a teacher."
From my understanding, the difference is the speaker you're addressing. If you are talking to your teacher John, you would say "ジョン先生" as an honorific when you address him. But that's not necessarily what's happening here. If you were talking about your friend John (ジョンさん), in that case, you would refer to him as normal because he just happens to be a teacher. He's not a teacher to you or a teacher at your school. Not 100% sure on this, but it makes sense in my head.
I think the down votes are from people who feel that Duo's answer sentence is wrong; —they think (either correctly or incorrectly) that it's instead meant to be the way I wrote it.
I was simply getting at out how saying "John-sensei is a sensei" seems funny to me, a bit like in English if we say "Dr Luke is a doctor".
I didn't mean that Duo was necessarily wrong to have marked the sentence I wrote as being correct though. Not all Dr Lukes are medical doctors; and not all ジョン先生 are 教師 (きょうし). Those sentences, assuming they are ever said, are still funny sounding nonetheless. ^^
You almost always need a particle after each term except at the end of a sentence. Of course there are other rules and practices. It is good though to always think about the proper particle to use.
In my Sxxxxxg Android phone the Japanese keyboard can be downloaded and I can switch to different keyboards with the globe sign button. If yours does not have it, download the Google one. https://play.google.com/store/apps/details?id=com.google.android.inputmethod.japanese
You pretty much always add some honorific to other people's names. You can leave it off if you're really, really close with someone, but even then just saying the person's name alone (without making it a nickname or using some affectionate honorific like くん or ちゃん) is rare.
How do I type the little backward "E"???
Depends what keyboard you are using...
Romaji input on mobile/desktop?
- Z Y O ⇒ ジョ
- J Y O ⇒ ジョ
- J O ⇒ ジョ
- X Y O ⇒ ョ
- L Y O ⇒ ョ
Kana input 12 key layout on mobile touchscreen?
- よ 小 ⇒ ョ
Kana input JIS layout on an English desktop keyboard?
- Shift+9 ⇒ ョ
Kana input NICOLA layout on my Japanese thumb-shift desktop keyboard?
- 変換+L ⇒ ョ