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.
estimate
Noun
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An estimate is a judgement of a size, value, amount, cost, etc., which is based on incomplete information.
British Telecom will give you an estimate of the cost before doing any work.
estimate
Verb
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If you estimate the number, size, value, amount, cost, etc. of something, you guess based on incomplete information.
It's difficult to estimate how long it will take to get to London because we don't know how bad the traffic will be.