I hope I understand your question correctly,
in Persian short vowels: /e/ (ِ ), /o/ (ُ ) and /a/ (َ ) are not usually written,
but the long vowels: /ā/ (ا), /ū/ (و) and /ī/ (ی) are written.
here are a few example:
در /dar/ (in)
دار /dār/ (gallows)
در /dor/ (pearl)
دور /dūr/ (far)
دیر /dīr/ (late
سر /ser/ (secret)
سیر /sir/ (garlic)