该 (gāi): that, such

It's already midnight. You should go home and sleep; you have to get up early for work tomorrow.

The doctor said his illness is serious and he should be hospitalized for treatment without further delay.

I am mainly to blame for this failure; I didn't prepare adequately in advance.

The sky is so dark; it looks like it's going to rain. Don't forget to take an umbrella when you go out.

When it's your turn to speak, just speak confidently and don't be nervous.