Ответ:

  1. Irregular plural forms: person – people; child – children; man – men; woman – women; tooth – teeth; foot – feet.
  2. Always plural: clothes – clothes; jeans – jeans; sunglasses – sunglasses.
  3. Uncountable nouns: help – no plural form; water – no plural form; tennis – no plural form; French – no plural form; Italian – no plural form; Spanish – no plural form; English – no plural form.