The possessive case is conversational shorthand for an “of” or “for” phrase: the bone of the dog is the dog’s bone; the birthday party for my daughter is my daughter’s birthday party. If you need to form a possessive noun, add an apostrophe and an “s.”
When you have two or more singular nouns that together possess something, add the apostrophe and “s” only to the last one: Mary and Ken’s mom.
The most important exception to this rule is the possessive form of the pronoun “it.” The possessive of “it” is “its”—no apostrophe. With an apostrophe, the word is a contraction (shortened form) of “it is.”
Keep in mind that an apostrophe attached to a noun indicates possession. Do not use an apostrophe to separate the “s” at the end of a plural: apostrophes, not apostrophe’s. The exception is when you want to indicate plurals of letters: abc’s.
Usually, you should add an apostrophe and an “s” to singular nouns that end in “s”: the bus’s morning route, the iris’s color, the boss’s secretary.
But some singular nouns with more than one syllable, especially if they contain an internal “s,” can sound awkward if you add another at the end. In those cases, add only the apostrophe: Damocles’ sword, Jesus’ teachings. Better yet, in these cases, avoid the possessive altogether: sword of Damocles, teachings of Jesus.
Singular objects, such as buildings, furniture, machinery and other inanimate objects usually are not modified to show possession: the keyboard of the computer is the computer keyboard; the door of the oven is the oven door; the room of the school is the school room; the leg of the chair is the chair leg.
Sums of money and periods of time, although they can’t literally possess something, are treated as though they can. For instance, the length of time designated by a week is a week's time; the amount of money earned in a month is a month’s salary; the value of that salary is (at least) a dollar's worth.