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The Mayfield Handbook of Technical & Scientific Writing
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Present Perfect Tense

Use the present perfect tense to express states or actions that occurred at an unspecified time in the past, were repeated in the past, or began at an unknown or arbitrary time in the past and continue into the present.

Beverage cans have emerged as the single most important market for aluminum. [unspecified time]

--William Hostold and John Duncan, "The Aluminum Beverage Can," Scientific American (modified)

Tomsk-7, whose existence was classified until about 1990, is thought to have poured and pumped about a billion curies of high-level waste, or 20 Chernobyls' worth, into lakes in the region and into underground formations. [repeated action]

--Tim Beardsley, "Lethal Legacy," Scientific American

For more than a century, researchers have known that exposure to high pressure can injure or kill. [past action that continues]

--Richard Moon, Richard Vann, and Peter Bennett, "The Physiology of Decompression Illness," Scientific American

To form the present perfect, use the present tense form of the auxiliary have and the past participle of the following verb. The perfect is commonly used with the following adverbs: already, always, ever, for, just, lately, never, recently, since, still, and yet.

Since World War II, chemists have released more than 70,000 new chemical compounds into the environment.

--"Take It or Leave It," Valley Comic News

Turnips have been cultivated for 4,000 years.

--"Take It or Leave It," Valley Comic News

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