Is it true that some irregular nouns change their vowel sound to form the plural?

That's not quite right! While some irregular nouns do have vowel sound changes in their plural forms, it's not a defining characteristic of irregular nouns.

Here's a better way to understand it:

* Irregular nouns are nouns that don't form their plural by simply adding "-s" or "-es". They have their own unique plural forms.

* Vowel sound changes can be a part of some irregular plural forms, but they're not the only way irregular nouns form their plurals.

Here are some examples:

Vowel sound change:

* Man - Men

* Foot - Feet

* Tooth - Teeth

Other irregular plural forms:

* Child - Children

* Woman - Women

* Mouse - Mice

* Goose - Geese

So, while vowel sound changes are sometimes a part of irregular plural forms, they are not the defining characteristic of irregular nouns.