The engine of English, grammar allows you to competently write and speak so that you are clearly understood from your audience. Since you will be judged by TOEFL iBT human raters in terms of whether you use grammar with a minimum number of errors during TOEFL iBT speaking and writing, you will need to have good control of your grammar. One area of concern is whether or not you can use verb tenses without distracting shifts. There are five rules you should consider when you are completing TOEFL iBT speaking and writing tasks.
Rule 1 : Do not randomly shift verb tenses in writing.
Incorrect: Jerry nodded, and the clerk looked at me with a sorrowful face. I smiled. Having arrived at the bus station only a few minutes prior to departure, we quickly begin to say our good byes. Jerry walks me to my car. Looking deep into one another’s eyes, we hold our last conversation before he left.
Revised: Jerry nodded, and the clerk looked at me with a sorrowful face. I smiled. Having arrived at the bus station only a few minutes prior to departure, we quickly began to say our good byes. Jerry walked me to my car. Looking deep into one another’s eyes, we held our last conversation before he left.
Rule 2 : In some cases, it is possible to use the present with the past.
I know that she went to the concert last week.
Rule 3: When using “by + time phrase,” use a past perfect or future perfect tense.
Incorrect: By 1990, she graduated from college.
Revised: By 1990, she had graduated from college.
Incorrect: By next week, I will finish my research paper.
Revised: By next week, I will have finished my research paper.
Rule 4: When using “since,” or “for + time,” use present perfect tense.
Incorrect: Since I was a child, I lived in this city.
Revised: Since I was a child, I have lived in this city.
Example: I have lived in this city for seven years. (Speaker still lives in the city.)
Example: I lived in this city for seven years. (Speaker no longer lives in the city.
Rule 5: When using “in + time,” use simple past.
Incorrect: In 2000, Tom graduate from college.
Revised: In 2000, Tom graduated from college.
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http://www.michaelbuckhoff.com/page14.html (TOEFL iBT Pronunciation)
http://www.michaelbuckhoff.com/page17.html (TOEFL iBT Grammar)
http://www.michaelbuckhoff.com/page10.html (TOEFL iBT Vocabulary)