1. Although a and one have similar meanings, they can hardly be used interchangeably.

2. When counting continuously, it is customary to replace one with one.

3. Using one before nouns often indicates quantitative comparison.

4. Used in some fixed phrases.

In a hurry, in a word

One by one, one day.

In addition:

Usage of infinitive articles a and one

1. As for "one", the concept of "quantity" expressed by indefinite article A is far less than the numeral one (

I want to live to be a hundred years old. I want to live 100 years.

(It is a kind of "gossip", so one is used instead of one)

A hundred days' journey is too accurate. -Mr swan. The trip lasted 100 days.

What is said here is true, so to replace one with one, you can add an adverb like completely.

Comparison:

Give me an apple, will you Give me an apple, will you

Give me an apple, will you Give me an apple, will you (instead of two, three ...)

We saw a panda in the zoo this morning. We saw a panda in the zoo this morning.

The answer is "What did you see in the zoo this morning?" Question, so the point is panda)

We saw a panda in the zoo this morning. We saw a panda in the zoo this morning.

The answer is "How many pandas did you see in the zoo this morning?" Question, so focus on one)

It has been raining for more than a week .. It has been raining for more than a week (maybe eight or nine days).

It has been raining for more than a week ... it has been raining for more than a week. (maybe two weeks).

2. Speaking of "same" or "same", the concept of "combination" expressed by A is not so clear and loud.

For example:

In the old society, the two industries would never be consistent.

We are all moving towards one goal, and that is communism.

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