You're fully occupied, and someone else is asking you for help, then you have to reject him. But how to say "I've no time" in Chinese? Only 5 minutes will make you learn the Chinese expressions.
wǒ yǒu shì zǒu bù kāi
I am tied up.
wǒ tuō bù kāi shēn a
I can't make it at that time.
nǐ kàn, wǒ zhèng máng zhe ne
You see I'm in the middle of something.
duì bù qǐ, wǒ yǒu qí tā ān pái le
I'm sorry, I'm otherwise engaged.
wǒ hái yǒu bié de shì yào zuò
I have other fish to fry.
zài kāi huì ne, guò huì ér dǎ gěi nǐ
I am in a meeting. I'll call you later.
duì bù qǐ, wǒ yǒu shì
Sorry, I have plans.
wǒ xiǎng, dàn shì wǒ hěn máng
I'd love to, but I am busy.
Source: China Daily