The central obstacle in mastering the word lies in the vowel order au. In English, the diphthong "au" can produce a wide range of vocal sounds, from the broad open sound in caught to the soft vowel in gauge. In modern spoken English, the phonetic pronunciation of restaurant typically collapses into two rapid beats, /ˈrɛs.trɑːnt/, or a hurried three beats, /ˈrɛs.tə.rɒnt/. Because the middle unstressed syllable is spoken as a nondescript schwa, the ear offers no useful clues about which vowel belongs in that slot.
To understand why the letters arrange themselves this way, one must examine dining establishment etymology. The word traces back to Paris in the mid-1760s. An entrepreneur named Mathurin Roze de Chantoiseau opened a shop serving concentrated, nutrient-dense meat broths designed to revive invalids. These medicinal broths were called bouillons restaurants, coming directly from the French root restaurer, which means "to restore" or "to refresh."
Latin: restaurare ("to rebuild, repair")
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Old French: restaurer ("to restore, revive")
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Middle French Participle: restaurant ("restoring, restorative")
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Modern Noun: restaurant ("a place providing food to restore energy")
The word crossed the English Channel during the late eighteenth century, preserving its exact French orthography. English kept the letters but drastically flattened the pronunciation. In French, every vowel in res-tau-rant receives deliberate enunciation. English speakers, by contrast, rush past the second syllable to slam into the final consonant, leaving writers to guess whether the "a" or the "u" comes first.
