I
You have more pens than I (have).
have可以省略,所以就寫I.
以下供參考：
用than引導的比較狀語從句
It’s easier than I thought.這比我想像的要容易.
She sings worse than she did before.她比過去唱得糟.
She speaks better English than I （do）.她的英語說得比我好.
It is raining worse than it did yesterday.今天雨下得比昨天更大了.
The weather was worse than I had expected.天氣比我預料的壞(糟).
I can throw the ball farther than you (can).我可以把球扔得比你更遠.
She is now happier than she has ever been.她現(xiàn)在比過去任何時候更快活.