present imperfect vs simple past

present imperfect

noun
  • A grammatical tense which presents the action in the present as continuous, not yet over 

simple past

noun
  • A tense used to describe something that happened in the past, formed by the inflection of a single word, without any auxiliary verb such as be or have. 

How often have the words present imperfect and simple past occurred in a corpus of books? (source: Google Ngram Viewer )