Special singulars with their plurals

Answer 1

The are different kinds of "special" singulars in the sense you mean. One kind is nouns that form the plural by changing the vowel instead of simply adding -s, as most plurals are formed. These include old words like man/men, goose/geese, mouse/mice etc.

Other old nouns have plurals in -n, such as oxen, brethren and children ( children actually has two pluralizing elements, the -r and the -n).

There are nouns that have the same form in the singulalr and the plural, which includes imported words like species, series and congeries; but also native animal words like sheep or deer.

Some nouns may have both regular and irregular plurals. Fish, for example, is generally fish in the plural, but may sometimes be fishes.

And some are always plural in form but singular in meaning, like economics and data.

Answer 2

1. Child - Children
2. Man - Men
3. Woman - Women
4. Tooth - Teeth
5. Foot - Feet