> However, only some vowels
Pretty sure it’s all vowels isn’t it?
“Thee”: apple, evening, island, offer, underpass
“Thus”: any other words where you don’t need to avoid a glottal stop to differentiate the word.
I realized long after the edit window expired that autocorrect changed it to “thus”.
[dead]
Hmm... I think an /u/ ("oo") triggers "thuh" for me, as in "the ubermensch." But that might just be my idiolect.