The word “debt” contains a mute letter.

在debt这个词中有一个不发音。

In a dictionary the words are arranged in alphabetical order.

词典里词是顺序编排。

Delta is the fourth letter of the Greek alphabet.

δ是希腊中第四个。

The heading was in large letters.

标是用大号印刷。

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

一个什么样五个词，当你拿掉两个后还有六个剩下？