158 have／had 表示推断

这是美国人的用法，但有时英国人也这么用。

have／had 主要是用在与 to be 连用时：

—There's a tall gred bird fishing in the river．

—It has to be／must be a heron．

—河里有一个大个的灰鸟在捕鱼。

—那一定是一只苍鹭。

had + to be 可表示说话人对过去可能发生的事认为是肯定无疑的：

There was a knock on the door．It had to be Tom．

有人敲门。肯定是汤姆。（他肯定是汤姆。）

had + to be 也可用来代替 must + 完成式：

—I wonder who took the money．

—It had to be Tom．／It must have been Tom．He's the only one who wasthere．

—我不知道谁把这钱拿去了。

—肯定是汤姆。当时只有他在那里。

但为了避免混淆，建议学生们还是尽量用 must 的各种形式。