Breakdown of Когда ей было тридцать лет, она была бедной студенткой.
она
she / it (for a feminine noun)
студентка
student (f)
год
year
быть
to be
Questions & Answers about Когда ей было тридцать лет, она была бедной студенткой.
Why do we use ей instead of она in the first part of the sentence?
In Russian, age is expressed by saying how many years have been assigned to someone. We use the Dative case for the person. So instead of saying she was 30, Russians say to her it was 30 (ей было тридцать лет).
Why is the verb было neuter in the first half, but changes to the feminine была in the second half?
In the first half, expressing age is an impersonal construction, so the past tense verb defaults to the neuter было regardless of who we are talking about. In the second half, она (she) is the active grammatical subject, so the verb must agree with her feminine gender: она была.
Why does this sentence use лет for years, and when would I use года instead?
The word for years changes based on the last digit of the number. Numbers ending in 5 through 9, and 0 (like тридцать) use the genitive plural лет. You would use года for numbers ending in 2, 3, or 4 (except for 12, 13, and 14).
Why does the sentence say бедной студенткой instead of the dictionary form бедная студентка?