Final answer:
An apostrophe indicates possession and forms contractions, but it is not used for emphasizing words or creating plural nouns (unless they also indicate possession).
Step-by-step explanation:
An apostrophe is used in English to indicate two main things: possession and the formation of contractions. Apostrophes are not used for emphasizing words or creating plural forms of nouns that do not also indicate possession. When indicating possession with a singular noun or an indefinite pronoun, you add an apostrophe followed by an 's'. In the case of plural nouns that end in 's', you place the apostrophe after the final 's'. For plural nouns not ending in 's', add both an apostrophe and 's' to show possession.
In contractions, apostrophes represent omitted letters, such as in the words 'can't', which stands for 'cannot', or 'it's', representing 'it is'. This functionality of apostrophes becomes particularly handy in informal writing and speech.