The positive effects of exercise affect people positively. — To affect (verb) means to influence or have an impact on something or someone. Effect (noun) means the result of being affected by something or someone; to effect (verb) means to bring something about, as in to effect a policy change (or to bring about a policy change as a result or effect).

This is just between you and me, but we're among the grammar nuts of the world. — Between is used for two or more subjects that are clearly distinct from each other, and among, for those that form part of a group. "The differences between Chinese, Greek, and Spanish are substantial. These can cause major confusion among the participants of the language conference."

A number of hours can be quite an amount of time. — It's pretty easy: if you can count it, use number; otherwise, use amount. Do distinguish the difference between amount of time (the indefinite continued progress of existence and events in the past, present, and future regarded as a whole), and number of times (instances or occasions of something happening or being done).

An apostrophe ('), like a hook, indicates possession. — To show that something belongs to one person, the apostrophe goes before the s (e.g. woman’s hair); to more than one person, after the s (ladies' room). Apostrophes are also used to indicate a contraction (wouldn't, replacing the missing o). Just as important, constructions like 1920s (in reference to the decade) and high 80s (temperature in degrees) don't need the apostrophe.

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