Pronoun Reference: ref  
EXPLANATION: 

When you use a pronoun, make sure it clearly refers to a specific word or group of words. If the pronoun appears to refer to more than one word, the sentence you make can be confusing. (Look at sentences 1 and 2 for example.) Confusion also emerges when the pronoun does not refer to any specific word. (Look at sentences 3 and 4 for example.)  
 

Vague Reference
Clear Reference
My sister argued with mom every time she was home for vacation. Every time when my sister was home for vacation, she argued with mom.  
OR

Every time when mom was home for vacation, my sister argued with her.

Jack told his father that he did not understand a thing. Jack told his father, “You don’t understand a thing.”
My father is an engineer, but I am not interested in it. My father is an engineer, but I am not interested in becoming one 

OR  

My father is an engineer, but I am not interested in engineering.

I have dreamed of becoming a great philosopher and am trying my best to achieve it. I have dreamed of becoming a great philosopher and am trying my best to achieve my dream.
  
NB: Always check pronoun reference when you use pronouns such as it, this, that, and which. A high percentage is that you are making a confusing or unclear sentence because the pronoun you use has no clear reference.
Also read University of Victoria's explanation regarding this error:
http://www.maclab.uvic.ca/writersguide/Pages/MarkProRef.html
 
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