您好，想請教一些問題，我造了一些句子，懇請幫忙看一下有無錯誤(不符美語表達方式、有更好的表達方式等)，謝謝您。

1.關於狗狗「牽繩」的英文

(1) Remember to put a leash on your dog when you walk it.

(2) Always keep your dog on a leash when you walk it.

(3) Walking dogs off leashes(a leash) can be dangerous.

→ 請問leash要用複數還是單數也可以？

2.關於「肺活量」的英文，我查到的是vital capacity(有其他說法嗎？)

(1) Swimming needs great vital capacity.

(2) I have weak/strong vital capacity. = My vital capacity is weak/strong.

→ 想表達我的肺活量很小/大。

3.關於單字或文章「沒背熟」、「背不起來」

(1) I know you had memorized the words, but you didn't remember them.

(2) This word is too long for a 5-year-old kid; he just can't memorize it.

(3) Because I didn't memorize this article well, I couldn't answer most of the questions (on the test).