Two words forming one noun: toothpaste, bus stop, mother-in-law
First word modifies second; stress usually on first word
Same structure
Same structure
💡 Plural usually on the last word: bus stops, toothbrushes. Exception: mothers-in-law.
✓I need to buy some toothpaste.
✗I need to buy some paste of tooth.
Compound noun, not 'of' phrase.
✓The bus stop is over there.
✗The stop of bus is over there.
Compound noun structure.
✓My mothers-in-law are visiting.
✗My mother-in-laws are visiting.
Plural on main noun in hyphenated compounds.