bygone vs past tense

bygone

noun
  • An event that happened in the past. 

adj
  • Having been or happened in the distant past. 

past tense

noun
  • A grammatical form (often a verb form) that refers to an event, transaction, occurrence, or object that happened (or had happened), or existed, at some time before now (the applicable reference time). 

  • A grammatical form (often a verb form) that is used in a conditional expression to refer to an event, transaction, occurrence, or object that is hypothetical and often counterfactual. 

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