The preposition "por"
(for, because of, by, on account of, for the sake of, on behalf of, through or along). 
The preposition "por"  has many meanings and several uses to express :
1)   Cause, reason or motive, to show obligation or concern for a person , and duty.  
2)   Support           
3)   Exchange          
4)   Place            
5)   Por = per (frequency, velocity, price)        
6)   To mean "by"           
7)   In certain set phrases:           
8)   Multiplication and measurements,        
9)   Distribution, proportion,          
10)  Periods of time: parts of the day, duration.        
11)  Means transportation or transmission        
12)  Substitution          
13)  Motivation          
14)  To express motion          
15)  Expressions that indicate an opinion held by someone         
16)  Movements          
17)  Emotions          
18)  Manner          
19)  "por"  is sometimes used with an infinitive        
20)  "por" is also used to show: aim        
In Spanish the preposition "por" is one of the most useful and common. 
The preposition "por"  sometimes is translated as "for," and so is the preposition "para", and  also often means "by". 
Think of "por"  as a preposition  usually indicates cause , motive, etc, rather than simply as a translation for "for.
Try to learn how "por"  is used rather than how it is usually translated. You will find easier to learn it in the long run.  
   If you find it difficult to differentiate between motivation >>>>>>>>>> (which is expressed with por)
   and purpose                                                                                            >>>>>>>>>>>   (which is expressed with para)
decide whether the idea could be expressed using the compound preposition “due to.”
 If that is what you are trying to say, use the Spanish preposition por.     
Motivation is the cause rather than the goal.       
The preposition para is used to express the idea of “in order to.”   
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