1. In indicates location inside or within something else.

Pattern 1: verb + in + noun
We live in that house.
The pencils are in the box.

Nouns commonly used after in:

Geographical areas:
city, continent, country, state, town
He lives in Seattle, Washington, in the U.S.A.

Comfortable, protected places:
alcove, large chair, cocoon, nest
He sat down in the chair and read his novel.

Inside areas:
attic, balcony, basement, building, corner, hall, kitchen, office, room She is in her office, in that building, in room 302, in the corner.

Vehicles where the passengers cannot walk around: car, canoe, helicopter, small airplane, small boat We went in the car, but they went in a helicopter.

Publications and speeches
article, book, dictionary, encyclopedia, lecture, magazine, newspaper, speech
He didn't say that in his speech, but I read it in the newspaper.

Pattern 2: be + in + 0 noun
He can't come to the phone because he's in bed.

Nouns commonly used after in:
bed, church, class, jail, place, school, town

Pattern 3: verb + noun + in + noun
Put the pencils in the box.

Typical verbs:
drop, get, insert, lay, place, push, put

Typical nouns:
bag, box, drawer, file, folder, notebook, sack, suitcase, trunk, wallet

2. In indicates membership of a group or category.

Pattern: be + noun + in + noun
There are seven people in our family.
Your brother is the expert in that office.

Nouns commonly used after in:
association, category, choir, chorus, club, family, fraternity, group, office, society, sorority

3. In can indicate a period of time.

Pattern: be + in + time period
century in + the She was born in the 1800s.