I'm a native Spanish speaker and I've been learning English for many years. They always taught us that there are two sets of vowels and we learned how to use them mostly by reading and practicing, no sort of rules or anything of that sort. I moved to the US a couple of years ago with my family, and my mom is trying to improve her understanding of English. Only after she started asking me about pronunciation did I notice how unnatural it feels to try to pronounce something for a foreigner. I've been taking classes since I was 4 probably, so it feels almost natural to me. So why isn't it only the vowels that sound like diphthongs, or the ones that sound like Spanish vowels?
closed as unclear what you're asking by Janus Bahs Jacquet, anongoodnurse, Centaurus, Ellie Kesselman, Nicole Apr 20 '15 at 15:06
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In potentially ephemeral comments, Professor Lawler wrote:
For non-low vowels, English is arranged in pairs of tense - lax in high and mid front and back. That's /i/ tense ~ /ɪ/ lax for high front (peach vs pitch); /e/ tense ~ /ɛ/ lax for mid front (rake vs wreck); and in the back, /u/ tense ~ /ʊ/ lax (suit vs soot) for high back; and /o/ tense ~ /ɔ/ lax (boat vs bought) for mid back.
Spanish, for instance, has only 5 vowels and does not distinguish tense from lax vowels, so those pairs are hard for many speakers of other languages to distinguish, let alone pronounce correctly.
Oh, and the reason is that the Great Vowel Shift moved long vowels to different places (long /i/ went to /ay/, for instance), but didn't move short vowels. The tense vowels are the formerly long vowels that moved up, and the lax vowels are the formerly short vowels that didn't move.
This answer has been made Community Wiki because it is John’s answer, not mine.