The document discusses verb tenses and how they are used to indicate when an action occurs. It explains that verbs in the present tense describe actions happening now, past tense for actions that already occurred, and future tense for actions about to happen. Examples are provided of rewriting sentences from present to past tense and vice versa by changing the introductory word (yesterday to today). The same is done to change sentences from past to future tense using the introductory word "tomorrow".