我完全明白你的意思。

Wǒ wánquán míngbai nǐ de yìsi.

I completely understand what you mean.

我完全不懂。

Wǒ wánquán bù dǒng.

I don't understand at all.

你不用担心，这件事我完全可以解决。

Nǐ bùyòng dānxīn, zhè jiàn shì wǒ wánquán kěyǐ jiějué.

You don’t need to worry, I can completely handle this matter.