I am not a teacher.
That's quite a big question.
Lax vowels are also called short vowels, and tense vowels are also called long vowels.
In one syllable words that end in a vowel, that vowel is typically tense.
In one syllable words that end in a consonant, the vowel is typically lax.
Compare, be - bet, say - sat, to - top, and so on.