Beginner level

(Difference 1) A determiner cannot have a comparative form.

• 
  Normal adjective: pretty > prettier
  

(Prettier is the comparative form of pretty.)

• 
  Determiner: that > [nothing fits here]
  

(There is no comparative form.)

(Difference 2) A determiner often cannot be removed from the sentence.

• 
  Normal adjectives removed: The young boy stole a silver watch. 
  

(This is grammatically sound with the normal adjectives removed.)

• 
  Determiner: The Young boy stole a silver watch. 
  

(The sentence is flawed with the determiners removed.)

(Difference 3) A determiner often refers back to something (i.e., it's like a pronoun).

• 
  Determiner: Release those prisoners immediately.
  

(The determiner those refers back to something previously mentioned. In other words, it has an antecedent (the thing it refers to).)