Use 'Either A or B' to say that there are exactly two options or outcomes: A or B. 'Either' emphasizes the fact that there are exactly two options.
- We can drink either beer or wine. (= We have only these two options: beer and wine.)
'Neither A nor B' is the negative form of 'Either A or B'.
- I can speak neither Chinese nor Spanish. (= I cannot speak Chinese. And I cannot speak Spanish)
- I can neither speak Chinese nor Spanish.
When you use either 'either' or 'neither', A and B must be of the same grammatical type:
- nouns/noun phrases: "We can drink either beer or wine."
- verbs/verb phrases: "Either come with me or go home!"
- prepositional phrases: "Come either with your parents or by yourself!"
- clauses: "Either you study or you fail the test."
However, sometimes you will see sentences whose options/outcomes do not seem to be the same grammatical type:
- We can either drink beer or wine.
'Drink beer' and 'wine' may seem like different grammatical types: a verbal phrase and a noun. However, 'wine' is actually short for 'drink wine.' In speaking, people often leave out the second instance of the verb:
- We can either drink beer or (drink) wine.
If both A and B are singular, the verb must also be singular.
- Neither he nor she knows how to solve the problem. (= "Neither he nor she know" would be wrong. The sentence is not "Neither they know" but "Neither he knows, nor she knows")
If either A or B is plural, the verb must also be plural.
- Neither the managers nor the employees know what to do. (= Here 'knows' (with an 's') would be incorrect, because the sentence is actually "Neither the managers know, nor the employees know")
