Question

1. I couldn't swim at all when I was young.

2. I couldn't swim at all because I was young.

(Are both grammatical? Which one is better?)
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3. My pencil is longer than you.
4. My pencil is longer than your pencil.
5. My pencil is longer than your one.
6. My pencil is longer than yours.
(#3 is wrong. Right? What about the others? Are the others grammatical? Which one is commonly used among them?)

Answer 1

1 and 2 are very similar, but not exactly the same. 1 states a time when an event happened (or didn't happen, in this case), and 2 gives a reason why that event happened.

4 and 6 are correct. 6 is probably more commonly used than 4.

3 and 5 are not. It's hard to explain ... just idiomatically incorrect.