"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).