Present perfect
Have / has +
past participle of verb
Uses:
a. Actions started in the
past and continuing
in the present
b. Unfinished period
of time
c. Unspecified period
of time
d. Experiences.
Gasoline has been
essential to our
transportation.
Many diseases have not
been eradicated yet.
Have vaccinations
impacted human health?
Past perfect
Had + past participle
Uses:
a. Actions finished before
other actions in past
Before the invention of
vaccines, smallpox had
killed millions of people.
Scientists thought
previous cultures had not
immunized themselves
against illnesses.
Had ancient cultures
eaten meat before they
discovered fire?
G
Adjuntos:
![](https://es-static.z-dn.net/files/d28/efe28471bece0818fb83646f26ffa7a6.jpg)
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