He replies, "I act because I must."

他回答说：“我这么做是因为我必须这么做。”

youdao

Because I must spend too much time doing my homework.

因为我要花太多的时间做我的家庭作业。

youdao

I have to go now because I must be punctual for class.

我现在不得不走了，因为我得准时去上课。

youdao

She said: "Because I am a woman, I must make unusual efforts to succeed.

VOA: special.2010.08.08

But even as black Baptist, I could have gone certain black Baptist and run out, because I must struggle against homophobia, and my profound love for gay brothers and lesbian sisters.

作为一个黑人浸信教徒，即使我去黑人浸信会，也会被赶走，因为我必须于恐同症作斗争，以及我对同性恋者们深深的爱。

普林斯顿公开课 - 人性课程节选

If I don't want this thing over here to be tortured, that must be because I think it's me.

如果我不希望这边的这块东西被折磨，那一定是因为我觉得那是我。

耶鲁公开课 - 死亡课程节选

For me writing an e-mail often takes quite a bit longer than a phone call because I must craft it much more carefully, people often read and reread e-mails multiple times.

FORBES: Young People, Stop Just Texting Me and Give Me a Call Once in a While PLZ! Part Deux

"Because of that I must teach my child that this is so and put that principle into practice and when my child persistently does wrong then I must show pain will follow persistent disobedience, " he added.

BBC: Boy with placard

Well, I must be a moron because I found it incredibly difficult.

FORBES: SP: Why Cadillac?