This document discusses the usage of the present perfect and simple past tenses. The simple past is used with specific past time adverbials to indicate remoteness, while the present perfect emphasizes the present perspective or result of an event rather than when it occurred. Various time phrases are categorized as being used with the simple past, present perfect, or either tense. The document also notes that context can imply past time without an adverbial and discusses using the past tense for completed vs. incomplete historical periods.