Comma before linking words

[mrmvp]

Do I have to write comma before and, or, and but? If they followed by a subject plus a verb plus an object

1. I am tall, but my brother is short.
2. Sarah is a teacher, and her brother is teacher too.
3. My sister has two cars, but the blue one is broke down.

In the following sentences, I won't use a subject plus a verb plus an object in the second half of the sentence .Do I have to use comma in the following sentences?

4. My computer is out dated and so is my brother' computer.
5. I am tall but my brother isn't.
6. My sister likes action movies but I don't.

 

[emsr2d2]

Do I have to write use/put a comma before "and", "or" no comma here and "but" no question mark here if they are followed by a subject plus a verb plus an object?

1. I am tall, but my brother is short. No comma required.
2. Sarah is a teacher, and her brother is a teacher too. No comma required.
3. My sister has two cars, but the blue one is has broken down. No comma required.

In the following sentences, I won't use a subject plus a verb plus an object in the second half of the sentence. space after a full stop but not before Do I have to use a comma in the following sentences?

4. My computer is outdated and so is my brother's computer. No comma required. You can also omit the final word.
5. I am tall but my brother isn't. No comma required.
6. My sister likes action movies but I don't. No comma required.

Note my corrections (in red) and comments (in blue and grey) above. The only one I might use a comma in is #2 so that the reader doesn't read the opening as "Sarah is a teacher and ...", thinking that another occupation might follow.