Englishfor English speakers
twice
Determiner
—
If something is done twice, it is done two times.
I have been to the shop twice today.
—
Two times: double.
He was working twice as fast as I was.
Twice three is six.
a
Determinative
—
A is used when the following word could be any of a certain type.
Compare "A book I saw on the shelf" and "The book I gave you yesterday".
a
Noun
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A is the first letter of the alphabet.
The letter "a" comes before "b".
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In some schools, an A is a very high grade.
Ron got an A on his earth science test.
week
Noun
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A week is a period of time that lasts seven days.
There are seven days in one week.