Learn Finnish: Hän ei ole lääkäri. Hän on opettaja. - She is not a doctor. She is a teacher.

In Finnish, when you make a verb negative, you use a negative verb like ei (is not) followed by a special base form of the main verb, called the connegative. For the verb olla (to be), that form is ole. You cannot use the regular positive form after a negative word, so you must say ei ole instead of ei on.

The translation says 'She', but how do we know hän means 'she' and not 'he'?

We do not know from this sentence alone! The Finnish pronoun hän is gender-neutral and translates to both 'he' and 'she'. The English translation uses 'she' just as an example, but without more context, this could equally mean 'He is not a doctor. He is a teacher.'

Where is the word for 'a' in 'a doctor' or 'a teacher'?

Finnish does not have articles like 'a', 'an', or 'the'. The word lääkäri simply means 'doctor', 'a doctor', or 'the doctor' depending on the context of the sentence.

Does the word lääkäri change its ending because the sentence is negative?

No, professions stay in their basic, dictionary form when you are simply stating what someone's job is or is not. Both lääkäri and opettaja are in their basic forms here, regardless of whether the sentence is positive or negative.