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?)

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.

1. "I couldn't swim at all when I was young." and 2. "I couldn't swim at all because I was young." are both grammatically correct sentences. However, there are slight differences in meaning and emphasis between the two.

1. "I couldn't swim at all when I was young." emphasizes that you were unable to swim during your youth, without explicitly stating the reason. It implies that your age was a determining factor in your inability to swim.

2. "I couldn't swim at all because I was young." places more emphasis on your age as the specific reason for your inability to swim. It suggests that being young directly caused your lack of swimming ability.

Regarding which one is better, it depends on the context and the intended emphasis. Both sentences are grammatically correct, so you can choose the one that best conveys your intended meaning.

3. "My pencil is longer than you." is grammatically incorrect. The correct form would be "My pencil is longer than your pencil," as shown in sentence 4. Here, you are comparing the length of your pencil to someone else's pencil.

5. "My pencil is longer than your one." is not commonly used in English. A more natural and common way to express this comparison is through sentence 6, "My pencil is longer than yours." Here, the possessive pronoun "yours" is used to mean "your pencil."

In summary, sentence 4, "My pencil is longer than your pencil," and sentence 6, "My pencil is longer than yours," are both grammatically correct and commonly used. The choice between them depends on the specific context and what you want to emphasize.