What is the noun formation of compound words?

There's no single "noun formation" rule for compound words, as they can be formed in various ways. However, here's a breakdown of the most common methods:

1. Combining Two Nouns:

* Simple Concatenation: The two nouns are simply joined together: *sunglasses*, *bedroom*, *toothbrush*.

* Hyphenation: A hyphen is used to connect the nouns: *high-five*, *brother-in-law*, *self-portrait*.

* Open Compound: The two nouns are separated by a space: *real estate*, *time zone*, *apple pie*.

2. Noun + Noun (Modified Noun):

* The first noun modifies the second noun, often indicating its purpose, material, or size: *mailbox*, *goldfish*, *toy car*.

3. Verb + Noun:

* The verb describes an action performed on the noun: *cookbook*, *playground*, *washing machine*.

4. Noun + Verb:

* The noun is the subject of the action described by the verb: *sunrise*, *heartbeat*, *eyewitness*.

5. Adjective + Noun:

* The adjective describes the noun: *blackboard*, *greenhouse*, *blueprints*.

6. Adverb + Noun:

* The adverb modifies the noun: *outreach*, *downtown*, *upstairs*.

7. Noun + Adjective:

* The noun modifies the adjective: *fireproof*, *waterproof*, *soundproof*.

Important Note: The noun formation of compound words can vary depending on the context and the evolution of language. Some words, over time, have become more closely integrated, with the hyphen sometimes disappearing.

Remember, these are just the most common patterns. It's best to consult a dictionary if you are unsure about the noun formation of a specific compound word.