In English, when two vowels are written together, you do not necessarily have to say both vowels.  This is called a vowel digraph.  One of the vowels is pronounced, the other is silent. This is the case in the word PAIN for the digraph AI which is pronounced as a long A while the I is silent.  Sometimes, the vowel digraph is pronounced as a vowel sound that you would not expect.  This is the case in the word VEIN with the digraph EI which is pronounced as a long A.

Long A can be spelled:  AI (pain); AY (bay); EI (vein)

Long E can be spelled:  EE (meet); EA (meat); EY (valley); IE (piece); EI (ceiling)

Long I can be spelled:

Long O can be spelled:  OA (boat); OE (toe)

Long U can be spelled:

Long EW can be spelled:  UE (revenue); OO (spoon); EW (flew); UI  (suit)

Long OY can be spelled:  OI (oil); OY (boy)

Long OW can be spelled:  OU  OW

Short o can be spelled:  AU (saucer); AW (law)

Check out these examples:

Beat:  B-long E-T

Feel:  F-long E-L

Eight:  Long A-T

New:  N-Long EW

Oil:  Long OY-L

Play:  P-L-Long A