ではなくてNot A but B, Conjunctive

When the auxiliary verb だ is seen in its negative conjugated form, it states that (A) is not correct, but (B) is. This form is ではなく, and may appear either with or without the conjunction particle て attached to the end.

ではなくて is commonly seen grouped with nouns, or other words that have been nominalized through the use of の or こと.

In ではなくて, the adverbial particle は is not strictly required. However, as this expression is comparing what (A) is 'not' to what (B) 'is', the contrast that は provides means that the phrase will sound far more natural with its inclusion.

Caution

て sometimes performs in a similar way to a 'comma' in Japanese, allowing structures to appear together that usually wouldn't. Due to this, if て is omitted from ではなくて, ではなく may not be linked directly to the next word, and a comma will be required. Regarding adjectives, this く form is known as the conjunctive form, and may only be linked directly with other 用言(ようげん) (conjugatable words), altering or adding to their overall meaning.