Origins
The English preposition 'after' conceals a surprising grammatical structure beneath its everyday simplicity: it is, etymologically, a comparative adjective meaning 'more off' or 'farther away.' Understanding this reveals how English builds complex temporal and spatial concepts from elementary spatial particles.
The word descends from Old English 'æfter,' which came from Proto-Germanic *after. This was formed from two elements: the root *af (off, away — the source of modern English 'off,' 'of,' and the prefix 'ab-' in borrowed words) and the comparative suffix *-ter (the same suffix seen in 'other' from *an-ter, and parallel to the Latin comparative suffix in 'alter'). The word literally meant 'farther off' or 'more removed,' and its original sense was spatial: the thing that is 'after' is the thing that is farther behind.
The PIE root behind Proto-Germanic *af is *h₂epo, meaning 'off, away from.' This root had enormous productivity across the Indo-European family. In Latin, it produced 'ab' (from, away) and the preposition 'post' through a related formation. In Greek, it gave 'apo' (ἀπό, from, away), the source of numerous English borrowings: 'apostle' (one sent away), 'apology' (a speaking away, i.e., a defense), 'apocalypse' (an uncovering, a removal of the veil), and 'apothecary.' In Sanskrit, it produced 'ápa' (away, off).
Old English Period
In Old English, 'æfter' was one of the most versatile prepositions in the language. It could mean 'behind in place' (walking after someone), 'later in time' (after the feast), 'according to' (after the law), 'in pursuit of' (go after the thief), 'in imitation of' (named after his father), and 'about, concerning' (ask after someone's health). Most of these senses survive in modern English, making 'after' one of the more semantically conservative prepositions.
The related word 'aft,' used in nautical contexts to mean 'toward the stern of a ship,' is simply a shortened form of 'after.' A ship's 'aft' section is the section 'farther behind' — the comparative meaning is still transparently spatial. 'Abaft,' another nautical term meaning 'behind' or 'to the rear of,' combines 'a-' (on) with 'baft' (a variant of 'aft').
The compound 'aftermath' offers one of the more charming etymological surprises in English. Modern speakers associate it exclusively with the consequences of disasters, but the word originally meant 'after-mowing.' The second element comes from Old English 'mæþ' (a mowing, a crop of grass), related to 'mow.' An 'aftermath' was the second growth of grass in a field after the first crop had been cut — a perfectly neutral agricultural term. The metaphorical extension to 'consequences that follow an event' appeared in the seventeenth century, and the negative connotation (consequences of a specifically bad event) solidified only in the nineteenth century.
Word Formation
Other 'after-' compounds are more transparent: 'afternoon' (the time after noon), 'afterthought' (a thought that comes after), 'afterlife' (life after death), 'afterbirth' (material expelled after the birth of a child), and 'aftershock' (a seismic event following the main earthquake). The prefix is productive enough that new formations continue to appear: 'afterparty,' 'aftermarket,' 'aftercare.'
The word 'after' also functions as a conjunction ('after he left'), an adverb ('they lived happily ever after'), and an adjective ('in after years'). In Irish English, it has developed a distinctive progressive construction — 'I'm after eating' means 'I have just eaten' — which calques the Irish Gaelic construction 'táim tar éis ithe.' This is one of the clearest examples of substrate influence on a variety of English.
Phonologically, Old English 'æfter' /ˈæf.ter/ has changed little: the main shift is the loss of the distinct 'æ' vowel (which merged with 'a' in most dialects) and the weakening of the final syllable. The word's two syllables and its stress pattern have remained constant throughout its recorded history.