The word “debt” contains a mute letter.

在debt这个词中有一个不发音母。

In a dictionary the words are arranged in alphabetical order.

词典里词母顺序编排。

Delta is the fourth letter of the Greek alphabet.

δ希腊母中第四个母。

The heading was in large letters.

标用大号母印刷。

It's very difficult! but I think it should be "hexad", and "hex" left when we take away the last two letters.

一个什么样五个母词，你拿掉两个母后还有六个剩下？