How do you know when to use "of" and when to use 's in English?

Answer 1

Generally, you use 's when referring to a person or living creature, or to proper nouns like place names, companies, organizations.

The boy's book

Jimmy's toy

Miami's beaches

The FBI's office in Washington

Using of is a little trickier. Technically it's correct with any noun. But it is rarely used in the following situations:

1. with the names of people. Example: "the house of Mary" is solid grammar, but it sounds strange in everyday speech. A native speaker would nearly always pick "Mary's house."
2. when referring to people with single nouns like woman, boy, or grandfather. "My grandfather's hand" is much more common than "the hand of my grandfather."

Otherwise, "of" can be used with any noun or noun phrase.

1. "the ear of the elephant" or "the elephant's ear"
2. "the slope of the mountain" or "the mountain's slope"