*Q: Play you tennis? Q1: Do you play tennis
*A: No, I play not. A1: No I don’t
The use of ‘do’ as a periphrastic auxiliary verb –the dummy ‘do’- in English is somewhat peculiar. We don’t find it where we think we should: in related Germanic languages, for example, such as Dutch, Swedish or German. It cannot be found in Romance languages such as Latin, French and Spanish either. Most of our neighbours use languages which would prefer forms like *Q and *A above; neither of which are acceptable in modern English which prefers the Q1/A1 forms with an auxiliary ‘do’.
Written evidence of the use of ‘do’ in unstressed affirmative statements dates back to at least the 13th century and appear in many examples in Shakespeare three centuries later:
‘The lady doth protest too much, methinks’ (Hamlet).
‘…whose horrid image doth unfix my hair.’ (Macbeth).
The use of ‘do’ in negative statements [A1: No. I don’t], stressed/emphatic declarations [We do use] and questions [Why do we do this? Q1: Do you play tennis?] are found from the 14th century onwards and these are the ones which have survived in modern English use.
The unstressed affirmative ‘do’ has only survived in some West Country dialects and it is here that we find clues to the origins of ‘do’ as an auxiliary in general. All three British Celtic languages (Welsh, Cornish and Breton) and Irish have a construction formed with ‘do’ and all four are connected by the Western seaways.
Welsh and Cornish, in particular, could be expected to have had their strongest influence on English speakers in the West Country. Place-name evidence suggests that Brythonic languages survived in some (non-Cornish) pockets here until the seventh century or later.
This bottom up influence of substrate languages primarily affects morphology, syntax and phonology, rather than lexis. In other words, the British learners picked up the English words correctly but put these together imperfectly using ‘creolised’ structures from their native language. Because auxiliaries are less complex than inflected forms they are especially common in regions where different languages come in to contact and positively encourage the ‘dropping’ of inflected verbs.
So why did it take so long (i.e. until the 13/14th centuries) for ‘do’ to show up in the written language? Simply because English was not a written language until then; everything was written in French or Latin. It was not until 1413 that English replaced French as England’s official language with the help, it is said, of three Cornishmen (John Cornwall, Richard Pencrych and John Trevisa) whose first language was Cornish and not English.