だけではなくて / だけでなく / だけでなくて

だけではなくて means "not only … but also": what comes before it is true, and the second half adds more. It follows a noun or the plain form of a verb or an い-adjective directly; a な-adjective keeps な.

A noun or な-adjective takes no だ in front: ×静かだだけではなくて. The plain form of a verb or い-adjective can be past or negative, whatever fits the sense.

は, て and じゃ

は and て can each be dropped, so the same point appears as だけではなくて, だけではなく, だけでなくて and だけでなく. The meaning does not change. With は it sounds a little more careful, and the forms without て suit writing; だけでなくて is a little more conversational. じゃ in place of では makes it casual.

In formal writing, ばかりでなく and the more literary のみならずtake the same place.

Particles around it

を and が drop on the first item: だけではなくて takes their place. When the second half adds a matching item, that item takes も, which replaces が or を there too. Other particles, such as に, stay in front of も.

• × 英語をだけでなく → 英語だけでなく
• × 勉強だけではなく、休むことが大切です → 勉強だけではなく、休むことも大切です

When the second half adds a new description of the same subject rather than a matching item, も is optional, as in the 静かな sentence above.