Why is the "sub" from "substance" pronounced in a different way than the one from "substantial"? [closed]
Solution 1:
"Substance" has a primary stressed initial syllable with a caret vowel. "Substantial" has an unstressed initial syllable with a reduced vowel, a schwa. Caret vowel is back; schwa is non-back. The vowel of "sub" in "substantial" can reduce despite being in a closed syllable, because "sub" is a Latinate prefix, so it is like the vowel of "con" in "constrained" in that respect.