Khan Academy
Khan Academy: Irregular Plural Nouns Foreign Plurals
You don't need to know how to speak Latin and Greek in order to understand English, but some words in English come from those languages. It's helpful to know how some Greek and Latin words change from singular to plural.
Sophia Learning
Sophia: Plurals Often Treated as Singular: Lesson 2
This lesson goes over singular words that are often misused as plurals. It is 2 of 2 in the series titled "Plurals often treated as Singular."