Englishfor English speakers
a
Determinative
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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.
heavy
Adjective
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Heavy is having a lot of weight.
This suitcase is very heavy.
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Having a lot of seriousness
day
Noun
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A day is a measure of time.
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24 hours
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Seven days make one week
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The time between midnight and the following midnight (or between sunset and sunset in Jewish reckoning)
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The time between sunrise and sunset, when it is daylight
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The part of a day spent at work or school
He spent two days at work means that on two days he went to work; he did not spend 48 hours at work.