"Are you a teacher?"
In Chinese definite/indefinite articles don't exist, however, as for a - an, it can be replaced with:
一 yī (number one, but also considered as the article a - an) + a classifier
note: all the chinese classifiers (they are around 150) serve every time you have to quantify things, in particular this 个 (gè) is used for people or abstract and general things, but also when it's not clear what is the correct classifier. I give you an example on how to use it:
八个人 - bā gè rén (eight people)
一个问题 - yi gè wèntí (a/one question).
in these cases you don't have to translate 个 (gè), just like in DL sentence, but remember that classifiers are mandatory in Chinese speaking of quantity.
note2: really, speaking about only people, when you have to espress politeness, the right classifier is 位 (wèi), so we could also say:
一位老师 - yī wèi lǎoshī (a teacher)
but be careful, it's not possible use it with 人 - rén (person).
教師 should also be accepted, I'm a native speaker and there are lots of variations on the term "teacher" in Chinese.
... I hate it, “一个” mean a/an but it have no need to be add to sentence to make it “你是一个老师吗？”, “你是老师吗？” also correct.
I wrote 他是一个老师 and still managed to be marked correct ☺️. I clearly missed 吗 and ?.
Technically the sentance is correct, but it would be more rude and sound like "Are you or are you not a teacher?"
I was so confused lol because when my tutor used to teach traditional Chinese to me and I used the old 'ma' form.