The two vowels ㅔ and ㅐ are usually not distinguishable when spoken by young people with a Seoul accent. Other parts of the country or older speakers may distinguish these two but don't worry about it. Of course for spelling you need to use the correct one.
 

W Semi-Vowels:
The three W Semi-vowels ㅚ , ㅙ and ㅞ are not distinguishable in spoken Korean but for spelling you need to use the correct one.
 

All the Korean vowels (except ㅔ and ㅐ ) have a distinct sounds. The problem for English speakers is that multiple vowels sounds can be represented by the same letter:

Vowel sounds sometimes represented with "u":

Vowel sounds sometimes represented with "o":

Vowel sounds sometimes represented with "a":

Non-native speakers often have trouble distinguishing between the plain, tense and aspirated form of the basic vowels. Here they are side by side:
 

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