1. USED WITH MONTHS
1.1. In January
1.1.1. We're going to Australia in January
1.2. In February
1.2.1. My birthday is in February
1.3. In March
1.3.1. The flowers start to bloom in March
1.4. In April
1.4.1. We celebrate Easter in April
1.5. The months always start with a capital letter
2. USED WITH YEARS IN + YEARS
2.1. In 1521
2.1.1. Hernán Cortés conquered the Aztec empire in 1521
2.2. In 1928
2.2.1. Alexander Fleming discovered the penicillin in 1928
3. USED WITH DECADES IN + THE + DECADES
3.1. In the '70s
3.1.1. Disco music emerged in the '70s
3.2. In the 1970s
3.2.1. Disco music emerged in the 1970s
3.3. In the seventies
3.3.1. Disco music emerged in the seventies
3.4. 70's
3.4.1. '70s
4. USED WITH CENTURIES IN + THE + CENTURY
4.1. In the 18th century
4.1.1. The Industrial Revolution began in the 18th century
4.2. In the eighteenth century
4.2.1. Benjamin Franklin was born in the eighteenth century
4.3. We always use ordinal numbers when referring to centuries
5. USED WITH SEASONS IN + SEASONS
5.1. In summer
5.1.1. It's hot in summer
5.2. In spring
5.2.1. She plants her garden in spring
5.3. In autumn
5.3.1. The leaves fall off trees in autumn
5.4. In winter
5.4.1. We love making snowmen in winter
5.5. The seasons are not written with capital letters
6. USED WITH A PERIOD OF TIME IN + PERIOD
6.1. In three hours
6.1.1. I'm going to meet Jane in three hours
6.2. In two years
6.2.1. We will finish the research in two years
6.3. In six weeks
6.3.1. She will give birth in six weeks
6.4. In five days
6.4.1. My package will arrive in five days
7. FOR PARTS OF THE DAYS IN + PART OF THE DAY
7.1. In the morning
7.1.1. We had breakfast in the morning
7.2. In the afternoon
7.2.1. He plays football in the afternoon
7.3. In the evening
7.3.1. They do their homework in the evening