We have many vowel sounds in English, not just a, e, i, o, u. These letters reflect written English, not spoken English.
To be more accurate in Spoken English, consider: peat, pit, pet, pat, put, putt, pot, pout, pole, pike, and pool. Say them out loud. Do you hear the difference?
We also have front and back vowels like feed and food. Feed is a front vowel, and food is a back vowel. This means the sounds are produced with the front part of the tongue raised towards the hard palate. Whereas a back vowel is the highest point of the tongue and is positioned relatively back in the mouth.
Also, for tongue height, consider feet and fat. If you do not feel the difference between high and low vowels with this pair of sounds, look at yourself in the mirror. The mouth is open wider for the vowel of fat than for the vowel of feet. The reason? The tongue is lower for fat.