Didn't Do Always Do What Do Does?

The humble, anomalous verb do is unusual in being both a lexical verb (basic meaning "to perform, bring about") and an auxiliary used in certain kinds of syntactic constructions. The Early Modern Period brought about the final stage in the series of changes that result in today's range of uses for "do."

First, study the senses outlined below;
Second, take the DIAGNOSTIC QUIZ linked at the bottom of the page.

As a lexical verb, do can be both transitive ("do my homework") and, more often, intransitive ("you have done well"). The basic meaning of the verb is general, though it produces many idioms with specific applications ("doing time," "do me").

It can also be a substitute for a previously expressed verb: "She ate sushi as quickly as he did," where did substitutes for "ate."

Additionally, there are five uses of do as a periphrastic auxiliary. As an auxiliary do does not have a specific meaning, nor does it change the meaning of the main verb; it has simply a syntactic function.

(1) The unemphatic affirmative first appeared in Middle English but became more common in later centuries. It is infrequent nowadays. It is especially useful in verse where an extra syllable might help the line scan, e.g.:

She looked at me as she did love
and made sweet moan.

In this example from Keats's "La Belle Dame sans Merci," the use of did love as opposed to the more normal loved provides an iambic foot at the end of the line. It does not change the meaning or add emphasis.

(2) The emphatic affirmative can add just the right touch to a petulant outburst: "I did finish the assignment!" It can express vehement emotion: "I do hope they can come!" 

An emphatic affirmative can often be made more emphatic by the addition of an adverb like "indeed" or "really." "I did indeed finish the assignment." "I really do hope they can come." Note that it would not make sense to add these adverbs to an unemphatic affirmative: "She looked at me as [if] she really did love" would change the meaning of Keats's line entirely. 

An important use of the emphatic affirmative is to mark a contrast with something already said: "I hated the book, although I did finish it." It can also mark a contrast with an assumption or an expectation that we might otherwise hold: "Laborers before the Industrial Revolution did have ways of protecting their livelihoods against new technology." "Carnivores do sometimes eat berries."

(3) If no other auxiliary is available, the interrogative  do steps in as a kind of dummy auxiliary at the beginning of questions:

"Do you dance?"

But note the other possible beginning auxiliaries: "Are we dancing?" "Have you danced?" "Shall we dance?" "Can you dance?" 

The interrogative do is used only if none of these others is available in order to avoid (it seems) constructions like *"Dance you?" 

(4) Negative sentences make use of do, again, if no other auxiliary is available:

"The train doesn't stop here anymore."

However, note that not every negative requires do: "The train has not left the station," "The train cannot blow its whistle," where another auxiliary is used with the negative. 

At times the interrogative do can be dropped for rhetorical purposes, as with JFK's famous "Ask not what your country can do for you . . ."  By leaving out "Do", Kennedy harks back to an earlier stage of English, especially the King James Bible of 1611, which has constructions like "Fear not" (rather than "Do not fear"). It gives the words more of a solemn, timeless feel.

(5) Do can be used in constructions that invert the order of the subject and verb. The paragraph above, for example, which speaks of the five uses of the periphrastic auxiliary do, has the following clause: "nor does it change the meaning of the main verb." Another example: "Well do I remember."

After reviewing the conditions, TAKE THE DIAGNOSTIC QUIZ ON "DO."