1. The document discusses the proper use of verb tenses in English sentences, including when to use past and present verbs together, how to use have/had correctly, and which tenses to use with different time expressions.
2. It provides examples of correct and incorrect usage and rules for determining whether a sentence with both past and present verbs is right or wrong.
3. Guidelines are given for using the present perfect, past perfect, simple past and present perfect tenses depending on if a time expression indicates past, present or a duration of time.