hard to understand vs impenetrable

hard to understand

impenetrable

adj

  • Opaque; obscure; not translucent or transparent. 

  • Not penetrable. 

  • Incomprehensible; fathomless; inscrutable. 

noun

  • A person not openly given to friendship. 

How often have the words hard to understand and impenetrable occurred in a corpus of books? (source: Google Ngram Viewer )