點解please之後一定係用present tense??

please confirm這種句子，英文用了祈使語氣（imperative Mood)
其實祈使句，許多時都會將主詞you省略，而且動詞要用原形動詞(bare infinitive)，如：
Be quiet!
Lend me your knife!
Let me do it!
等都將You省略，又因為祈使句動詞要用原形動詞，故
就要說Please confirm ......
而不會說Please confirming, please to confirm，甚至也不說Please confirms等
希望幫到你！

This is an informal speech and the sentence structure will rather than a formal one ( Subject + Verb + Object). Instead, we would just say :

Please + verb (verb form)
We don't need to use greund or infinitive for "the first verb" , that's it.
Hope you understand!
e.g.
1. Let's "do" it
圖片參考：http://l.yimg.com/f/i/tw/ugc/rte/smiley_4.gif
Let you doing/to do it X
2. Please keep quiet
圖片參考：http://l.yimg.com/f/i/tw/ugc/rte/smiley_4.gif
Please keeping/to keep quiet X

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!