After работать and after past or future forms of быть and стать, the profession or role takes the instrumental case: -ом/-ем for masculine, -ой/-ей for feminine, -ами/-ями in the plural. In the present tense with no verb, the job stays in the dictionary form instead.
Examples
- Я работаю инженером в большой компании.
- I work as an engineer at a big company.
- Она хочет стать врачом.
- She wants to become a doctor.
- Раньше он был учителем.
- He used to be a teacher.
- Моя сестра — переводчица.
- My sister is a translator.
The mistake to avoid
Adding как ("работаю как инженер") on the model of English "work as" — Russian just uses the instrumental.
Where this fits in
Работа и учёба, Russian B1: Talk about your job, workplace, and studies; give reasons and opinions.