Use the present perfect tense to express states oractions that occurred at an unspecified time in the past, were repeated in thepast, or began at an unknown or arbitrary time in the past and continue into the present.
Beverage cans have emerged as the single most importantmarket 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 tohave poured and pumped about a billion curies of high-levelwaste, or 20 Chernobyls' worth, into lakes in the region and into undergroundformations. [repeated action]
--Tim Beardsley, "Lethal Legacy," Scientific American
For more than a century, researchers have known thatexposure to high pressure can injure or kill. [past action thatcontinues]
--Richard Moon, Richard Vann, and Peter Bennett, "The Physiology ofDecompression Illness," Scientific American
To form the present perfect, use the present tenseform of the auxiliary have and the Since World War II, chemists have released more than70,000 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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