Sentences with irregular nouns

In English grammar, there are regular nouns (that obey to certain rules when you transform them in their plural) and irregular nouns. The irregular nouns change completly, don't change at all or don't obey to any specific rule, here are some examples:

My child is in the school./ My children are in the school.

There is a mouse in the kitchen./There are mice in the kitchen.

I broke my feet riding bycicle./ I broke my foot riding bycicle.

The man in the room is making me nervous./ The men in the room are making me nervous.

The woman in my reading club is a very good friend of mine./The women in my reading club are very good friends of mine. 

When I arrived home I had to change my clothes. (The noun "clothes" does not change in his singular and in his plural)

The fish we ordered was delicious. (The noun "fish" does not change in his singular and in his plural)

Yesterday at the mall I bought a pair of pants. (The noun "pants" does not change in his singular and in his plural)