Origins
The word 'do' is one of the most important verbs in English, serving both as a full lexical verb meaning 'to perform, to carry out' and as an auxiliary verb in questions, negations, and emphatic statements. It descends from Old English 'dōn' (to do, to make, to act, to cause, to put, to place), from Proto-Germanic *dōną, from PIE *dʰeh₁- (to put, to place, to set, to make).
The PIE root *dʰeh₁- is among the most productive roots in the family, though its reflexes have diverged so far in meaning that the connections are not obvious. Greek 'tithénai' (τιθέναι, to put, to place) produced 'thésis' (θέσις, a placing, a proposition — whence English 'thesis'), 'théma' (θέμα, something placed down — whence English 'theme'), and 'thḗkē' (θήκη, a repository — whence English 'apothecary,' 'bibliotheca'). Sanskrit 'dádhāti' (places, puts, makes) is the direct cognate. Latin 'facere' (to make, to do) is connected through the suffixed form *dʰh₁-k-, giving English an enormous vocabulary: 'fact' (a thing done), 'factory' (a place of making), 'fashion' (a manner of making), 'effect' (a doing out), 'perfect' (thoroughly done), 'sacrifice' (a making sacred), 'condition' (a setting together), 'edifice' (a building — from aedis + facere).
The most distinctive feature of English 'do' is its use as an auxiliary verb — so-called 'do-support.' Modern English requires 'do' in questions ('Do you understand?'), negations ('I do not understand'), and emphatic affirmations ('I do understand'). This is typologically unusual: no other major Germanic language uses 'do' this way. German forms questions by inverting subject and verb ('Verstehst du?' — understand you?), and negates with a simple particle ('Ich verstehe nicht' — I understand not). Dutch and Scandinavian languages work similarly.
Old English Period
The rise of do-support is one of the most studied and least resolved problems in the history of English. In Old English and early Middle English, 'do' was occasionally used as a causative auxiliary ('he did them go' meaning 'he caused them to go') and as a periphrastic substitute for a lexical verb. During the 15th and 16th centuries, do-support expanded dramatically, first appearing in affirmative declarative sentences (where it has since retreated to emphatic use only) and then becoming obligatory in questions and negations. By Shakespeare's time, both old forms ('Know you this man?') and new forms ('Do you know this man?') coexisted, but by the 18th century, do-support had become mandatory in most contexts.
The lexical verb 'do' also produced important derivatives. 'Deed' (from Old English 'dǣd') is 'that which is done.' 'Doom' (from Old English 'dōm') originally meant 'a judgment, a decree' — something placed down authoritatively — before it narrowed to mean 'fate' or 'ruin.' 'Deem' (from Old English 'dēman,' to judge) is related to 'doom.' These words preserve the older sense of 'do' as 'to place, to set, to establish' rather than the more general modern sense of 'to perform.'