begin vs happen

begin

verb
  • To come into existence. 

  • To be in the first stage of some situation 

  • To start, to initiate or take the first step into something. 

noun
  • Beginning; start. 

happen

verb
  • To occur or take place. 

  • To do or occur by chance or unexpectedly. 

  • To encounter by chance. 

How often have the words begin and happen occurred in a corpus of books? (source: Google Ngram Viewer )