For my regular column at Macmillan Dictionary Blog, I’ve been writing about flat adverbs and how our use of the word do has changed since Early Modern English.
I’ll start with the latter. Much ado about ‘do’ summarises the main uses of this complicated verb, then considers how modern usage compares with Shakespeare’s. Here’s a short excerpt:
Sometimes auxiliary do is inessential but included anyway. In ‘Conscience does make cowards of us all’, from Hamlet’s famous soliloquy, it is semantically superfluous, since the meaning of Conscience makes cowards of us all is basically the same. But do in this position was common in Shakespeare’s time, as Lane Greene notes. Nowadays it often serves to emphasise the verb following it – see sense 3 in Macmillan’s entry.
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Next up: Is adverbial ‘deep’ used wrong? is a defence of flat adverbs – adverbs that look just like their associated adjectives, such as deep and wrong. The resemblance leads to some muddled thinking and misguided claims:
Confusion is especially likely when the two words are synonyms. No one would argue that They arrived late should be They arrived lately, because late and lately mean different things. But people often claim that Drive slow should be Drive slowly, even though both are fully grammatical. Some insist that I’m good should be I’m well, though the meanings here differ a little. On a recent newspaper article about commonly misused words […], a reader commented that the phrase used wrong should be used wrongly because ‘an adverb is needed’.
Such groundless complaints are typical of amateur language policing. I describe it as ‘adoption of a half-remembered rule that was dubious to begin with’, followed by dogmatic repetition of the rule without bothering to look it up first.
When you assume you’re an expert, you’re apt to go wrong.
Sentence first 
I’ve been puzzling this very weekend how the rule that only auxiliaries could be inverted to form questions was so strong that a redundant “do” was introduced with declarative clauses so that the rule could be maintained.
The fact that the forms “I do go” and “Go I?” were both still in use in early modern English makes much more sense of this.
That was good timing! The auxiliary do and the ways it behaves are curious features of English. The Economist article I link to says the structure emerged as a result of Welsh influence, according to McWhorter’s Our Magnificent Bastard Tongue. Eventually I will read the book.
Pretty doubtful: McW has a bee in his bonnet about Celtic influences on English, though he’s not the only one. In particular, the same construction (though not mandatory, and often stigmatized) is found in Low German, Standard German, and Danish, and analogous constructions (at least for negatives) in the Finnic languages.
Ah. Then its occurrence in Finnic and other Germanic languages points to different possibilities. I’ll keep an open mind about it. Thanks for the information, John.
One of my classes yesterday was practicing past simple. A noticeable number of students answered the question ‘Where did he go?’ with ‘He did go to the park’. In fact, forming past simple with ‘did V’ would make the teacher’s and students’ task a *lot* easier than the regular/irregular system we’ve actually got!
Just before I read this, I answered a question on ELL Stack Exchange, to the effect of “which is correct – ‘he did leave’ or ‘he did leaves’?'”. I don’t know where the student got ‘he did leaves’. I could more understand a student asking “which is correct – ‘he did leave’ or ‘he left'”.
Just after I read this, I read a verse from one of the psalms in the KJV translation, ‘He rode upon the cherubims and did fly’.
PS many students were getting confused about auxiliary ‘do’ and lexical ‘do’ (the same happens with ‘have’). If I was designing a language, I would choose auxiliary verbs which were not also lexical verbs (except I know in the case of English, the auxiliary verbs grew out of the lexical verbs).
PPS Whenever the formula ‘how do you do?’ crops up, I tell my students not to try to understand the meaning of it – it’s just a formal way of saying ‘how are you?’. (And the answer is not ‘how do I do *what*?’)
It’s true: Where did he go? → He did go makes much more apparent sense than He went. Do is a very tricky verb for non-native-English speakers to get a handle on. It would be tempting to keep auxiliary and lexical verbs distinct, as you suggest, if one were to design a language.
Adverbial do is still used in West Indian creolese, e.g. ‘He does wash he hand all de time’, ‘He does be washing’ he hand…’
Thanks for letting me know about this. Note that this is auxiliary do, not adverbial. Irish English has a similar construction to ‘does be washing’, which it uses for marking habitual aspect.