The document discusses the use of the words "for", "since", and "ago" when talking about periods of time in the past, present, and future. It provides examples of how to use each word correctly in sentences. "For" is used to indicate a period of time from the past continuing to the present. "Since" refers to a point in the past continuing to the present. "Ago" is used with past tense verbs to refer to a time before now. The document provides exercises for readers to practice using these words accurately in different temporal contexts.