When “awhile” is spelled as a single word, it is an adverb meaning “for a time” (“stay awhile”); but when “while” is the object of a prepositional phrase, like “Lend me your monkey wrench for a while” the “while” must be separated from the “a.” (But if the preposition “for” were lacking in this sentence, [...]

In English, you can use an apostophe to:

indicate that one or more letters were dropped in a contraction:
it is > it’s
we are > we’re
does not > doesn’t
of the clock > o’clock
indicate possession:
a) singular with ’s
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An illusion is a misleading perception, usually visual. You see something, but you consistently misjudge its length, shape, motion or direction. To avoid further confusion, illusions are distinctly different fromhallucinations – which involve sensing something that is not actually present. In contrast, illusions deal with stimuli that are actually present, but they are misinterpreted or hard to interpret.
For [...]

“People who have specialized knowledge about a
scientific or technical issue are systematically
excluded from juries for trials where the issue is
relevant. Thus, trial by jury is not a fair means of
settling disputes involving such issues.”
1) What will be the inference?
2) What conclusion can you draw from this argument?
3) Which one of the following, if true, most [...]

So if you think you are not good in sentence correction and need some good quality exercise then nothing is better than OG-11 (Official Guide-11) Review. It has 138 questions of Sentence Correction. Answer key is very descriptive. After all you can never doubt their grammar skills as every person who prepare for GMAT purchase [...]