Notes on Sentence Parallelism

This principle of parallel construction requires that expressions of similar content and function should be outwardly similar.

Formerly, science was taught by the textbook method, while now the laboratory method is employed.

Formerly, science was taught by the textbook method; now it is taught by the laboratory method.

The left-hand version gives the impression that the writer is undecided or timid; he seems unable or afraid to choose one form of expression and hold to it. The right-hand version shows that the writer has at least made his choice and abided by it.

By this principle, an article or a preposition applying to all the members of a series must either be used only before the first term or else be repeated before each term.

The French, the Italians, Spanish, and Portuguese	The French, the Italians, the Spanish, and the Portuguese
In spring, summer, or in winter	In spring, summer, or winter (In spring, in summer, or in winter)

Correlative expressions (both, and; not, but; not only, but also; either, or; first, second, third; and the like) should be followed by the same grammatical construction. Many violations of this rule can be corrected by rearranging the sentence.

It was both a long ceremony and very tedious.	The ceremony was both long and tedious.
A time not for words, but action.	A time not for words, but for action.
Either you must grant his request or incur his ill will.	You must either grant his request or incur his ill will.
My objections are, first, the injustice of the measure; second, that it is unconstitutional.	My objections are, first, that the measure is unjust; second, that it is unconstitutional.

The position of the words in a sentence is the principal means of showing their relationship. The writer must therefore, so far as possible, bring together the words, and groups of words, that are related in thought, and keep apart those which are not so related. The subject of a sentence and the principal verb should not, as a rule, be separated by a phrase or clause that can be transferred to the beginning.

Wordsworth, in the fifth book of The Excursion, gives a minute description of this church.	In the fifth book of The Excursion, Wordsworth gives a minute description of this church.
Cast iron, when treated in a Bessemer converter, is changed into steel.	By treatment in a Bessemer converter, cast iron is changed into steel.

Source: Elements of Style by William Strunk, Jr.