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.
certain
Adjective
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If you are certain of something, you believe it to be true without doubt.
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If something is certain, it will happen without doubt.
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If you make certain of something, you check that it is true or correct.
certain
Determiner
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Used to talk about groups or individuals without saying exactly which one(s).
Certain problems can be solved with very simple math.
Her job gave her a certain freedom.
Certain of the members did not agree.
ratio
Noun
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A ratio is the comparison in size or number between two things. It is written mathx:y/math and read mathx/math to mathy/math. It is the same as the fraction math\frac {x} {y}/math.
The ratio of men's jobs to women's is 8 to 1.
But Germany now has one of the highest ratios of spending to GDP in the European Community.
The low student/teacher ratio makes this a great school.