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.
density
Noun
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Density is how much stuff fits in a particular area.
The population density of Singapore is one of the highest in the world.
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The density of a material is how closely packed its atoms are.
The density of water is higher than the density of steam.
function
Noun
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Something or someone's function is its purpose or its job.
The army sometimes performs a policing function.
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A function is an organised event.
I met Sylvia at a church function last week.
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If mathX/math is a function of mathY/math, then mathX/math exists because of or is affected by mathY/math
The speed of the computer is a function of its main processor.
When you see, mathY=f(X)/math, you read it, "mathY/math is a function of mathX/math". Here the value of mathY/math depends on the value assigned to the variable mathX/math.
function
Verb
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If something is functioning, it is working and is not completely broken.
Her heart stopped, but her brain continued to function.
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If something functions as mathX/math, it does the job of mathX/math.
In this sentence, "sentence" functions as the subject.