Breakdown of Она часто смотрит на себя в зеркало.
она
she / it (for a feminine noun)
часто
often
в
in / to (direction)
на
on, at / by (transport) / to (a place) / to (in answer to)
Questions & Answers about Она часто смотрит на себя в зеркало.
Why do we use себя here instead of неё?
Себя means "oneself." Whenever the person doing the action (the subject) is the same as the person receiving the action, Russian uses себя. If you said Она смотрит на неё, it would mean "She is looking at her" (meaning a completely different woman!).
What case is себя in this sentence?
It is in the Accusative case. The verb смотреть (to look) paired with the preposition на (at) requires the target of the looking to be in the Accusative case. The Accusative form of себя is just себя.
Why is it в зеркало (Accusative) instead of в зеркале (Prepositional) if the mirror is a location?
Russian treats the act of looking into something as a direction (where are you directing your eyes?). When prepositions like в (in/into) show direction, they take the Accusative case. Зеркало is a neuter noun, so its Accusative form looks exactly the same as its dictionary form.
The infinitive is смотреть. Why does the ending look like an -ить verb (смотрит)?
You caught an exception! Смотреть is one of the special verbs that end in -еть but follow the second conjugation (the -ить pattern). That is why it conjugates as она смотрит instead of using an -ет ending.