The more complicated meaning of one is as an indefinite pronoun. A pronoun is a word used to replace a person's name. In some sentences one can replace you. This use of the word refers to any person in general or to all people. Using one in this way is considered very formal in US English, but is more commonplace in UK English. In the US, the indefinite pronoun one is often replaced by you. However both words are correct in both countries. The first example sentence may seem overly formal in US English.
One must not eat in the laboratory.
You must not eat in the laboratory.
When a sentence uses one as a number, a different pronoun should be used to avoid confusion.
Awkward and confusing:
One needs to practise one's second-language for at least one hour a day.
One needs to practise one's second-language for at least an hour a day.
We need to practise our second-language for at least one hour a day.
You need to practise your second-language for at least one hour a day.
It is sometimes considered poor style to use both the indefinite pronoun one and a definite pronoun (his, her, mine) in the same sentence. The second sentence below is not wrong, but the switch between the general one and the specific his appears, to some people, to be awkward. Which does not mean that the structure is wrong, or never used, just that some people don't like it.
There is no greater love than to lay down one's life for one's friend.
There is no greater love than to lay down one's life for his friend.
In very formal British English one is also used as the first person pronoun instead of I. British royalty now often use one in place of the royal we. (The royal we is used, by royalty, to replace I with we.) That means if the current queen wished to express her displeasure at a political cartoon, she might say, 'One is not amused!' instead of 'I am not amused.' or 'We are not amused.' Using one or we instead of I should only be done if your name is Elizabeth and you are a Queen.
The word an is the original form of one. The word one is used to represent anything or person. However in the US the pronoun one is usually best avoided, the same way one as a replacement for I should be avoided. Next week I will look at stranger uses of the word one.