不得不,只好;只得;必须;不得已;
例句:
You don't have to knock─just walk in.
不必敲门——进来就是了。
Do you have to hum so loudly? (= it is annoying)
你非得用这么大嗓门哼唱不可吗?
There has to be a reason for his strange behaviour.
他的古怪行为一定事出有因。