Negative sentence [closed]

The direct negative of "either" is "neither".

However, the meaning of your sentence, "either of the boys is present" is ambiguous.

"Either" is almost always followed by "or"

Either, conjunction

used before the first of two alternatives (the other being introduced by “or”)

However, "either" is not always followed by "or", usually when it is not denoting two mutually exclusive alternatives.

For example:

Either option is okay.

Meaning, each option is indeed okay; "either" is used instead of "each" to imply that while either could be used, only one will be.

If your sentence means that each boy is present, the correct sentence and its negation would be

Each of the boys are present / None of the boys are present.

If you sentence means that one of the boys is present, but the other one is not:

Either this boy, or that boy is present. / Neither this boy, nor that boy is present.