I have learned lax vowels and tense vowels. And then, if there is a stress in a vowel, is it always pronounced as a tense vowel?
What do you native English speakers think? Thank you so much as usual in advance.
Hans51if there is a stress in a vowel, is it always pronounced as a tense vowel?
No, no, no. 
'apple' has a stressed but lax "a".
'empty' has a stressed but lax "e".
'simple' has a stressed but lax "i".
'coffee' has a stressed but lax "o".
'ugly' has a stressed but lax "u".
CJ
Comments
Thank you so much.
I will change the question a little bit.
Do tense vowels always have a stress in prounciation?
What do you think? Thank you so much as usual in advance.
Not always, no. For example, the final "o" in "silo" or "halo" is tense, but the syllable with the "o" is unstressed. The same pattern is used in words like "follow" and "mellow". Likewise, the "u" in the last syllable of "vacuum" is tense, but that syllable is unstressed.
"relay" as in "relay race" has a tense "a", but that final syllable is not stressed. "create" has a tense "e", but that first syllable is not stressed. "ally" as in "Canada and its allies" has a tense "i", but that final syllable is not stressed.
I think if you took a large sample and analyzed it statistically, you might find that tense vowels are more likely to be stressed, whether with primary or secondary stress, but we can't say that tense vowels always take the stress. There are too many cases where that "rule" doesn't work.
CJ