And to know what personal pronoun use to substitute a noun you just use your english knowlege
I'll give you the equivalents, you might use the following table, it basically can help you get the hang of it:

Singular:
Я - I
Ты - you
Он - he
Она - she/it
Оно - it

Plurals:
Мы - we
Вы - you(plural)
Они - they

Студенты - who? Students -> they -> они -> будут.
Преподаватель - who? A teacher -> he -> он -> будет.