记住，‘mice’的单数形式是‘mouse’。

Remember that the ___singular of 'mice' is 'mouse'.

‘Feet’是复数形式；它的单数形式是‘foot’。

'Feet' is plural; its ___singular form is 'foot'.

在这个句子里，‘book’这个词是单数形式。

In this sentence, the word 'book' is in the ___singular.