Below are the most important rules for capitalising words in English.
A common noun is the name for the people, places and things around us, such as woman, cat, tree, table, church, air, river, room, etc. Common nouns can also name non-visible 'things' such as idea, luck, happiness, memory, justice, etc.
These words are not capitalised in English (although they are in German).
Mary and I are no longer friends.
The grammar test was very easy.
Where did you buy your iPad?
* A proper noun is a name that identifies a particular person, place or thing.
The names of people: The new student is called Sadako Ishii.
The names of countries and continents: Everyone knows that China is the largest country in Asia.
The names of pet animals: I have a dog called Spot.
The names of towns and cities: My grandparents live in London.
The names of planets: The Earth is much smaller than Jupiter.
The names of rivers: The longest river is the Nile.
The names of lakes: Is there a monster in Loch Ness?
The names of streets: I live in Oak Road.
The names of buildings: Have you ever visited the Sears Tower?
The names of mountains: The highest mountain is Mount Everest.
The names of businesses: I think Apple computers are best.
The names of organisations: My mother works for the United Nations.
The names of sports teams: Do you like the Lakers?
The names of days/months: I was born on 2 April 1999, a Monday.
Note: Seasons are not capitalised. E.g., My favourite season is spring.
The names of holidays / festivals: Which is your favourite holiday: Christmas or Easter?
The names of periods of time: Life was hard and short in the Middle Ages.
The names of religions: The most common religion in India is Hinduism.
The names of languages / nationalities: Can you speak Russian?
Note: Languages and nationalities are always capitalised, both when used as nouns and when used as adjectives. (The French are a proud people. - I love French wine.)
My mother asked, "Where have you been?"
I saw President Obama in Macdonalds yesterday.
Have you met Doctor Spock?
Barack Obama is the first black president of the USA.
Spock is a doctor at UCLA.
Do you like living in the South?
There are many car factories in the Northeast
I saw a flock of birds heading south.
Just then Mother called me on my iPhone.
The man at the edge of the photo is Uncle Pete.
Have you met my mother?
The man at the edge of the photo is my uncle Pete.
I was born in St Martins Hospital.
My brother's in hospital after an accident.
Our next car will be a Mercedes.
I got an Acer notebook for my birthday.
The Pacific Ocean is the largest body of water on Earth.
Which is the largest ocean?
"If you listen, you will learn," the teacher said. "And you will not get a detention."
"If you listen," the teacher said, "you will learn."
In the two sections above (Easy rules / Harder rules), it is correct to use the word rule. For example, you must capitalise the first word of a sentence, proper nouns and common nouns when they are part of names. You must not capitalise other common nouns or other internal words within the sentence.
But there are many other situations where it is not correct to speak of rules: the writer can decide for herself which words to capitalise - or needs to follow a style guide determined by the organisation for which she works. (Students need to follow the "rules" given by their teachers.)
Below are a few examples of words that may or may be capitalised, depending on personal preference (or an organisational style guide, or teacher requirement).
The words within headings and book or film titles
Sections or divisions of organisations
Concepts, ideas, philosophies
Places or geographical features
Times of day
After a colon
An example of a style guide is the one used by writers for the Guardian newspaper. This link will take you to the style instructions under the letter C. (Scroll down to "Capitals".) The guide introduction notes the clear trend (in British English at least) towards using lower case.