I put my review in a blog entry before the actual shop updated :)

These are some of the mistakes that you can work on:

• 1) "Myungsoo, can I play your guitar?" the girl's beautiful hazel eyes twinkled... → Capitalize 'T' in "The girl's beautiful hazel eyes..." 
  
  2) ...her slender index finger pointed at the black guitar case, which was lying flat on the grass in front of them. → Do not put a comma before "which." It's a dependent clause. 
  
  3) Myungsoo countered as he opened up the case and took out his black acoustic guitar. → "Countered" is not the clearest word to use here. "Questioned" or "asked" is good enough.
  
  4) "Are you laughing at me?" Jiyeon said in mock outrage. → "Said" should be changed to "asked" or the like since she's asking a question.
  
  5) ...trying not to smile and failing miserably. → Use "but" instead of "and."
  
  6) He laughed heartily at her words, clapping his hands. → He laughed heartily at her words WHILE clapping his hands. 
  
  7) He liked the way she her lips so adorable → He liked the way she liked her lips so ADORABLY.
  
  8) ...she told him as she positioned his guitar in her lap. → she told him as she positioned his guitar ON her lap
  
  9) His mouth gaped open and shut → His mouth gaped open THEN shut
  
  10) ...seeing her fingers sliding across the fret board... → Seeing her fingers SLIDE. Don't suddenly use present tense.
  
  11) The boy kept his eyes closed, a smile stringing the corner of his lips. → "Stringing" does not fit the sentence at all. Change it to "forming" or something like such instead.
  
  12) Her face and eyes lit up like glory → This is certainly awkward, to be frank. Please use the thesaurus or your own mental vocabulary to change it. 
  
  13) The stars and the moon above knows I have fallen in love with you. → Star and Moon put together is plural; therefore, “knows” should be “know."
  
  14) The stars blinked, watching them kissing → The verb tense is wrong. “Kissing” should just be “kiss.”