Origins
The verb 'teach' is one of the most culturally important words in any language, and its etymology offers a profound insight into how early societies understood the transmission of knowledge. The word descends from Old English 'tǣcan,' which meant not 'to instruct' in the modern sense but 'to show, to point out, to demonstrate, to declare.' The teacher, in the original Germanic conception, was not a lecturer filling empty minds with information but a guide who pointed out what was already there to be seen.
Old English 'tǣcan' came from Proto-Germanic *taikijaną, a causative verb meaning 'to cause to see' or 'to show,' derived from the noun *taikną (a sign, mark, token — the source of English 'token'). The Proto-Germanic forms trace back to the PIE root *deyḱ-, meaning 'to show' or 'to point.' This root had an extraordinarily productive life across the Indo-European family. In Latin, it produced 'dīcere' (to say, speak — originally 'to point out,' source of 'dictate,' 'dictionary,' 'predict,' 'verdict'), 'digitus' (finger, the thing that points, source of 'digit' and 'digital'), 'index' (pointer, from 'indicāre,' to point out), and 'docēre' (to teach — yes, the Latin word for 'teach' comes from the same PIE root as the English one, though through different pathways). In Greek, *deyḱ- produced 'deiknynai' (to show), 'deigma' (a sample, something shown — source of 'paradigm'), and 'dikē' (justice, custom — originally what is 'shown' or 'pointed to' as right).
The connection between English 'teach' and Latin 'docēre' (to teach) is particularly elegant. Both descend from *deyḱ-, but through different suffixed forms: the Germanic branch took a form meaning 'to show' (from a noun meaning 'sign'), while the Latin branch took a causative form meaning 'to cause to know' (from a form meaning 'to accept, to be fitting'). Latin 'docēre' gave English 'doctor' (originally a teacher), 'doctrine' (a teaching), 'document' (something that teaches or proves), and 'docile' (easy to teach). Thus 'teach' and 'doctor' are cousins, and both ultimately mean 'one who shows.'
Old English Period
The past tense 'taught' comes from Old English 'tǣhte,' with the irregular vowel change that characterizes a small group of Old English weak verbs (compare 'think/thought,' 'buy/bought,' 'seek/sought'). The pronunciation shift from 'tǣhte' /tæːxte/ to 'taught' /tɔːt/ involved the loss of the velar fricative /x/ with compensatory lengthening and rounding of the preceding vowel, a regular sound change in English.
The word 'token,' as noted, is the nominal relative of 'teach' — from Proto-Germanic *taikną, meaning 'a sign' or 'a mark.' A token is literally something shown, a visible sign that stands for something else. The Old Norse cognate 'teikn' meant 'a sign, mark, or omen,' and the Gothic 'taikn' meant 'a sign' or 'wonder' (used in the Gothic Bible to translate Greek 'sēmeion,' the signs and wonders performed by Jesus). German 'zeigen' (to show, point) and 'Zeichen' (a sign, mark) are the modern German reflexes of the same root.
The semantic development from 'show' to 'teach' occurred within Old English itself. Early uses of 'tǣcan' emphasize the visual and gestural: pointing something out, drawing attention to something, demonstrating. By the late Old English period, the word had acquired the broader pedagogical sense of systematic instruction. The older meaning 'to show' was gradually taken over by other verbs — 'show' itself (from Old English 'scēawian,' to look at) and 'demonstrate' (from Latin) — leaving 'teach' specialized for educational contexts.
Later History
The phrase 'teach someone a lesson' preserves an interesting ambiguity between instruction and punishment, reflecting the historical entanglement of teaching with discipline. 'That'll teach you' is purely punitive, with no educational content — yet the verb is 'teach.' This usage dates to at least the sixteenth century and plays on the old understanding that experience (including painful experience) is a form of showing or pointing out truth.
In modern English, 'teach' retains its position as the plain, direct word for education, resisting displacement by Latinate synonyms like 'instruct,' 'educate,' and 'edify.' A teacher teaches; an educator educates; but the monosyllabic Anglo-Saxon word remains the default, the word children learn first and use most. Its etymological meaning — to show, to point — endures as perhaps the best description of what effective teaching actually is: not filling a vessel but pointing toward what matters.