Why is it sing, sang, sung — but never bring, brang, brung? And why is the song you sing the very same word as sing, just wearing a different vowel?
Most English verbs make their past tense the lazy way: bolt -ed on the end (walk → walked). A few hundred do something stranger and far older — they change the vowel in the middle of the word. sing · sang · sung. drive · drove · driven. These are the strong verbs, and the vowel-swap isn't random English messiness: it is ablaut, a system of vowel alternation inherited intact from Proto-Indo-European, six thousand years deep. Here it is, made runnable: a wall of verbs caught changing from the inside, the seven shelves the strong verbs sort onto, the o-grade hiding inside everyday nouns — and the slow, measured tide that keeps dragging these verbs back toward -ed.
Each row is a strong verb in its three principal parts — present, past, participle — the vowel doing the work the suffix does elsewhere. Touch one for the ablaut grades underneath, and the everyday noun that froze out of the same root. The last two rows are imposters: they look strong, but that final -t gives them away.
The strong verbs aren't a junk drawer of exceptions. Germanic philology sorts them onto seven shelves, and what decides the shelf is simply what follows the root vowel — a glide, a nasal, a liquid, a single consonant. Each shelf has its own fixed vowel pattern across the four old principal parts. Pick a shelf.
The vowels shown are the reconstructed Proto-Germanic pattern (the asterisk marks reconstructions). Modern English forms have been battered by later sound changes — the Great Vowel Shift above all — so today's drive/drove/driven looks less tidy than its ancestor *drīban / draif / dribun / dribans. The shelf is about the deep pattern, not the modern spelling.
Here is the quiet astonishment. The same vowel-swap that turns sing into sang also turns the verb sing into the noun song — song is literally the o-grade of sing, frozen into a naming word thousands of years ago. Tap a verb to wake the noun that ablaut left behind.
Strong verbs are a closed, shrinking club. No new verb ever joins by birth — google, text, photoshop all went straight to -ed. And the old members keep defecting: help was once strong (healp, holpen), climb was clomb, chew was chew/chowen. One careful study put real numbers on the leak.
The tide isn't quite one-way. A few verbs that were always weak grew a strong-looking past by sheer analogy — the pattern still has a faint pull. (How faint: each of these is contested or regional, and none is a new class.)
Here is the test that separates a true strong verb from an irregular pretender. A strong verb marks its past with a vowel change and no suffix. A weak verb — even a wildly irregular one like brought or kept — always hides a dental suffix, a -t or -d, however much the vowel also moved. Find the -t/-d and you've found a weak verb in disguise.