Articles explain the difference

Why are the articles different in these pairs of sentences:

1a) We had breakfast at 8 o’clock.

b) The breakfast on the train was awful.

2a) We left in the afternoon.

b) We left at night.

3a) We travelled by train.

b) We came by the early train.

4a) the Himalayas

b) Mont Blanc

5a) the United States

the Phillipines

b) England

6a) the London Road

b) London Road

7a) The telephone is a great invention.

b) Mobile phones are a nuisance.

—————————————————–

PDF version for easy printing: DifferencesBetweenArticles