The spelling of "wast taught" is influenced by the English language's complex history of sound changes, which have affected many words throughout its development. In this phrase, the "a" in "wast" is pronounced as a long vowel sound /eɪ/ and the "au" in "taught" is pronounced as a shorter vowel /ɔː/. The word "wast" is the archaic second-person singular past tense of "be," and "taught" is the past tense of "teach." The spelling of the phrase reflects its historical usage, with "wast" being an obsolete form of the verb "be" and "taught" representing a standard spelling of the word that has been in use since Middle English.
The phrase "wast taught" is a contraction of "wast thou taught". "Wast" is the second-person singular past tense of the verb "be", which means "were". "Thou" is the archaic second-person singular pronoun for "you". And "taught" is the past tense of the verb "teach", meaning "learned or instructed".
In terms of etymology, "wast" comes from the Old English word "wǣre" (past tense of "beon", an old form of "be"). "Thou" is derived from the Old English word "þū". Lastly, "taught" traces back to the Old English word "tǣhte" (past tense of "tǣcan", the old form of "teach").