Many sentences contain series — lists of similar things, actions, or people:

The nursing-home residents’ eyes light up when we set out chips, cookies, or brownies.

Series go wrong when the items in the list are mismatched — that is, not parallel:

To correct a mismatched series, make all the elements serve the same function in the sentence.

Sometimes items in a series are connected by either . . . or or neither . . . nor. These items should also take a parallel form:

Either form is correct; what’s incorrect is the lack of parallelism. The sentence can be revised in either of two ways:

Many prefer either playing cards or watching movies.

Many prefer either to play cards or to watch movies.