［英语写作概要］Chapter 12: Verbs: Consistency and Voice

Once you have decided whether to write in the present tense or the past tense, you must remain consistent. You cannot shift tenses in the middle of a sentence or the middle of a paragraph unless there is a reason within the context of the writing to make a change in time. In other words, you can't go from the past to the present and back to the past if all the events you are describing occurred in the past. You must remember when you are editing to check the tenses to be sure that you have been consistent with the choices you've made. Correcting shifts is easy. Simply choose one of the tenses and make everything with in the sentence that tense.

　　Voice describes the subject of the sentence. When the subject of the sentence does the action of the sentence, it is considered active. When the subject of the sentence does not do the action with in the sentence, the subject is considered passive. Passive voice is wordier and weaker than active voice so you should avoid using it whenever possible.