3 sentences for 快 (kuài — sharp)

1. 这把菜刀磨得很快。

   zhè bǎ cài​dāo mó de hěn kuài.

   This kitchen knife is very sharp.

   Source: Dong Chinese Dictionary

2. 这把剃刀不快了，得杠一下。

   zhè bǎ tì​dāo bù kuài le, děi gàng yī​xià.

   This razor isn't sharp anymore; it needs to be stropped.

   Source: Dong Chinese Dictionary

3. 他脑子快，经常能出好点子。

   tā nǎo​zi kuài, jīng​cháng néng chū hǎo diǎn​zi.

   He is quick-witted and often comes up with good ideas.

   Source: Dong Chinese Dictionary