Learn English – either + singular + ‘have’ or ‘has’

For any word or phrase that implies a set (e.g. variety, assortment, multitude, group, collection, lot, etc.), the best way to decide whether to treat it as singular or plural is to ask yourself "am I referring to the set's members, or to the set itself?" If you're referring to the set's members, use plural. If you're referring to the set itself, use singular (because, even though a set contains multiple things, it is a single thing).

Some examples might help to clarify this distinction. Imagine a florist conversing with one of her customers...

Example 1:

Customer: I'd like to have a bouquet sent to my wife for our anniversary. Would roses be a good choice?

Florist: Certainly, but consider mixing some daisies and lilacs in with the roses. A variety of flowers makes a lovely anniversary bouquet.

Note the use of the singular verb makes in the last sentence. Since the set is being treated as singular, we assume that the florist is making a statement about the set rather than its members. In other words, it's the variety, not the flowers, that she's identifying as that which makes a lovely bouquet.

Example 2:

Customer: I'd like to have a bouquet sent to my wife for our anniversary. Would roses be a good choice?

Florist: Certainly, but roses aren't your only option; you could send daisies or lilacs instead. A variety of flowers make a lovely anniversary bouquet.

The last sentence is exactly the same as the previous example's, except that the singular verb makes has been replaced with the plural make. The grammar is still correct, but the meaning has changed. This time, the florist is saying that any of the set's members can be used to create a lovely bouquet. She's not suggesting that the bouquet should contain the whole set (as in the previous example).

So both forms are correct, and the one you should use depends on the message you're trying to convey.

It's worth noting that many native English speakers-- myself included-- often mess this up, so you'll probably have to use context to figure out what we mean most of the time. Plural verbs seem to be preferred when the necessity of distinguishing between the set and its members is absent or unclear. In fact, you could probably get away with always using plural verbs. This is somewhat ironic since singular verbs have the advantage of not conflicting with the indefinite articles a and an (e.g. a variety, an assortment), which are exclusively singular. But for some reason, we prefer to break the rules in this case.

One reason may be the verb's close proximity to the noun that identifies the set's members (e.g. the flowers in "a variety of flowers"). Since that word is plural, it "feels right" to follow it with a plural verb, even if the actual subject of the sentence (the set) is singular.

Another reason may be that phrases like a variety of, a lot of, an assortment of, etc. are often used as substitutes for adjectives like various, many, some-- words that typically describe plural subjects and are therefor accompanied by plural verbs (e.g. "various flowers make lovely bouquets"). When people make this substitution, they may not feel comfortable changing the verb to its singular form because in their minds, they're still referring to multiple things as opposed to one.

So there is some gray area, but most of the time you'll do well to follow the rule described at the top of this post.

Note on definitions: The word variety has several meanings, of which the most relevant to the current topic is "diverse set". When it's preceded by the word the, as in "the variety of books on the table is unbelievable", it usually means "diversity" or "type". Those meanings have their own usage rules which are beyond the scope of this answer.

Both neither and either are used with singular verbs.

Ex. Neither of you is leaving until you have finished your work.

Ex. Either day is convenient.

BE CAREFUL when you use a compound subject with either or neither. Look at these examples below:

Ex. Neither my father nor my sisters like to play soccer.

In this sentence the plural verb like agrees with the subject that is closest to it = sisters=plural.

Ex. Either a melon or oranges are on the menu today.

The subject nearest to the verb is plural=oranges and so the verb is plural=are