The Instrumental has several different uses.
When to Use the Instrumental
1) To indicate the instrument that was used to perform an action.
We ate ice cream with a spoon.
2) To show the manner in which something is done.
The man spoke in a quiet voice.
3) With time expressions (e.g. parts of the day or parts of the year) to express “in” or “at.”
The nurse works at night.
4) To indicate the complement of a verb of being or condition.
Казаться to seem
Оказывается, to turn out to be
Оставаться to remain
Становиться to become
Считать to consider someone something
5) With the verb “to be”
The verb “to be” either takes a nominative or an instrumental complement. In the present tense, when the verb is not expressed, the complement is always in the Nominative case, as we saw in Chapter 1.
Он - студент.
He is a student.
Она - преподаватель.
She is a teacher.
To express a condition of being in the past, use the instrumental. Consider this sentence and notice how the instrumental and nominative are both used.
В прошлом году мой сын был студентом; теперь он преподаватель.
Last year my son was a student; now he is a teacher.