One; ones

 "One" and "ones" are pronouns used to replace a noun or a noun phrase that is understood from the context. They are used to avoid repetition and make sentences more concise. Here's a chart to help you understand their usage:

Pronoun	Usage	Example
One	To replace a singular countable noun when the noun is understood	I like the red dress, but the blue one is prettier. (one = dress)
Ones	To replace plural countable nouns when the nouns are understood	I bought some new shoes; the old ones were worn out. (ones = shoes)

Exercises: Fill in the blanks with "one" or "ones."

1. I need a new pen; this _____ is out of ink.
2. Can you pass me the green apple? I don't want the red _____.
3. She has several books, but her favorite _____ are the Harry Potter series.
4. We need to replace the broken chair with a new _____.
5. My socks have holes in them; I should buy some new _____.

Answers:

1. I need a new pen; this one is out of ink.
2. Can you pass me the green apple? I don't want the red ones.
3. She has several books, but her favorite ones are the Harry Potter series.
4. We need to replace the broken chair with a new one.
5. My socks have holes in them; I should buy some new ones.

These exercises should help you understand the differences between "one" and "ones" and how to use them correctly in sentences. Practice using these pronouns in various contexts to become more comfortable with them.

You generally do not use "a" with "one" when "one" is used as a pronoun to replace a singular countable noun. Using "one" as a pronoun already implies a single item, so adding "a" would be redundant.

Examples:

1. Correct: I like the red dress, but the blue one is prettier. (one = dress) Incorrect: I like the red dress, but the blue a one is prettier.
2. Correct: We need to replace the broken chair with a new one. Incorrect: We need to replace the broken chair with a new a one.

However, when "one" is used as a number or to indicate quantity, you can use "a" with "one."

Example:

• She has a one-year-old child.

In this case, "one" is used as a number to indicate the age of the child, and "a" is used as an article before the compound adjective "one-year-old."