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Paula went on a trip and just got back from her vacation. she is in class now but she couldn't focus on her text book. because she is thinking about her wonderful vacation in Florida. 

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* Correction :     Student's sentence        Corrected sentence        Grammar explanation       Better sentence    

Paula went on a trip and just got back from her vacation. she is in class now but she couldn't focus on her text book. because she is thinking about her wonderful vacation in Florida. 

Paula went on a trip and just got back from her vacation. she is in class now but she couldn't focus on her textbook. because she is thinking about her wonderful vacation in Florida.

Textbook is always correct. Text book is incorrectly used by people instead of the compound word 'textbook'. The plural form is 'textbooks'.

After Paula went on a trip she couldn't focus on her textbooks for the reason of thinking of a wonderful vacation in Florida.