"Either" is grammatically singular, even though it always refers to a choice between (usually) two things. That mismatch — a word that means "one of two" but takes a singular verb — is exactly why this one trips people up.
"Either" means "one or the other" — a single item, even when it's introducing a pair. Treat it exactly like you'd treat "one": singular verb, no exceptions for the fact that two things are being discussed.
Because "either" is almost always followed by a plural-feeling phrase — "either of the options," "either of us," "either of the twins" — your ear latches onto the plural noun right before the verb and wants to match it. The subject of the sentence is still "either," not "options" or "us," so the verb has to agree with "either," not with the noun sitting next to it.
When "either" is connecting two different subjects directly ("either X or Y"), the verb agrees with whichever subject is closer to it:
If "either" is functioning as the subject of the sentence by itself (not directly paired with "or" between two named subjects), treat it as singular — "is," "was," "has" — no matter how plural the rest of the sentence feels.
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