て-form: な-Adjective で JLPT N5
The connective で that joins clauses, the copula's て-form.
Formation
な-Adjective / Noun + で: 静かで, 学生で.
Explanation
で is the て-form of the copula, used to join clauses the way a verb's て-form does (静かで、きれいです, 'it's quiet and clean').