有關tense 的問題

你係想問用
I WANT you這個動詞的tense是現在式是否正確
我估你的問題暗示由於want要做的事是下午(將來)...

其實文字或句子是溝通邏輯，動詞的時態是配合講述行動的當下。

我i講的時候就是我want你的時候，要做的事是將來是將來的事；其實要做的事的動詞是take care，在這講述中那是沒有時態的(即不是受時間規管了的行為)，而是infinitive詞。

講完要回答的
附帶講一句
I want you to do xxx.
是指令句。而且只會由上司/主人向下人講的，可能你會見啲家長業個菲僱每件事都咁講，便以為這是正確英文，但其實英語人只會在很激動或者在僱員/傭人點極都唔明時才會用這詞句。

如何請人幫你忙或者指示別人但叫人受落的講法是另一條問題。

對,不過你可以寫：Mary 係今朝早十點個陣問Mandy"Mandy,can you help me take care of my child from 3:00p.m. to 4:00p.m. on today afternoon,please？ 這樣較禮貌

啱 ar.............

Mandy, would you mind taking care of my child from 3:00pm to 4:00pm this afternoon?

This will be more polite. You are asking for a favor from somebody, so it's better to ask if the person is available to do something for you at a certain timeframe.

2013-11-22 04:34:02 補充：
Starting the sentence by "I want you" sounds like a command.

冇錯呀，因為Mary「而家」想Mandy下午幫佢湊仔，所以用present tense係岩架。

不過你衣句後面就累贅左少少，你可以寫:
Mandy, I want you to take care of my child from 3 to 4 this afternoon.

仲有，正如上面講，語氣重左啲，你可以話:
Mandy, may I ask you to take care of my child from 3 to 4 this afternoon?

"on today afternoon"

改成 today

或 this afternoon

或極其量 today afternoon

如果你講 in the afternoon today 或 today in the afternoon 就太累贅

因為明顯 pm 已知是 afternoon (or night)

沒有問題，除咗冇乜禮貌之外，基本上正確。