/j/ Palatal approximant vs. /dʒ/ Palato-alveolar affricate.

 Example "young" /jʌŋ/ vs. "Jung" (philosopher) /dʒʌŋ/

For Spanish speakers it is problematic because the Spanish pronunciation of words beginning with "y" such as "yo", "ya", etc. can often be either. In English you have to be very careful when distinguishing between them. Look at your text book for further details.