The apostrophe is used to indicate third-person proper noun possession, or to indicate a contraction. (Or, occasionally, to indicate glottal stops in languages other than English.) It is not used when indicating a plural. It is not used when writing the simple passive present tense of a verb, or the active future tense.
So these are just fine:
The exception here is the word "it". The possessive form applying to "it" is "its". That is to say, "Where's Tony's dog? That's its ball right there!" is correct, while "Where's Tony's dog? That's it's ball right there!" is not.
The following are incorrect uses of the apostrophe.
|He grew up in squalor, in the slum's district of town.||The slums are a location. One slum, many slums. The district is not possessed by the slums - it is comprised of them.|
|[His home] became like an oven in the summer, an ice locker in the winter, and during the rain seasons's flooded to the point they could leave nothing on the floor for fear it would be swept away.||Seasons. Plural. Otherwise, the question would be, "The rainy season's what?"|
|His mother kept them isolated and away from the eye's of the rest of town. She kept her feature's hidden for her work...||Plural. Eyes. Features. Not rocket science.|
|As the year's progressed, he mastered the cantrip's...||Plural! Plural!|
|...aware of a group of burly young boy's sneering and hurrying after him.||*whimpers*|
|...could only train him the rudimentary basic's of such a weapon...||Plural! PLURAL! Kyaaaieee!|
|Perhap's Bezrel may have been disgusted by...||... the word is, in fact, "perhaps". It's not even plural. How could you do this?|
(As you may have guessed, this bit was engendered by one individual and their writing. It makes me stabby.)
Tune in next time for Capital Letters: When Not to Use Them!