Subject-Verb Agreement – Are These Two Agreements Correct?

The moon, as well as the stars, was shining down on them. is grammatically correct.

The commas are probably considered optional by some writers and grammarians, but they're helpful for indicating that as well as is a preposition here, so I'd say that they're necessary. M-W online shows this: ": in addition to : besides (the coach, as well as the team, is ready)". Merriam-Webster's Dictionary of English Usage recommends using the commas when the first subject is singular.

When as well as is used as a conjunction, it joins two elements without using commas, just as it would were the sentence The moon and the stars were shining down on them. M-W online shows this: ": and in addition : and (brave as well as loyal)".

Many native speakers make what are called "proximity errors". Many usage experts disagree on what "subject-verb agreement" means in sentences that have complex subjects with a singular and a plural element.

He as well as you are right is an example of a proximity error: "Under the 'proximity rule', the verb is governed by the element nearest to it."

See Merriam-Webster's Dictionary of English Usage pages 140-141, as well as, for a thorough discussion of the problem.

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