Final answer:
The idea that a compound noun should be treated as separate words if not found in a dictionary is false; instead, one should consider standard usage and guidelines, which may indicate that it should be hyphenated or combined.
Step-by-step explanation:
To determine if a compound noun is written as two separate words, one word, or with a hyphen, checking a dictionary is a practical approach. If the compound noun is not listed in the dictionary, the statement suggests treating it as separate words. This is false. Nouns that are not found in the dictionary may still be valid compound nouns. Additionally, some compound nouns may change form over time or vary between different styles of English. When in doubt, apart from consulting the dictionary, consider common usage, rules, and intuition about whether the compound should be separate, combined, or hyphenated.
For example, as per the guidelines that exist for forming compound words, when two words serving as modifiers precede a noun, they should be hyphenated to make the relationship clear, such as in 'well-known author' and 'chocolate-covered peanuts'. But if the modifiers come after the noun, they shouldn't be hyphenated, as in 'the peanuts are chocolate covered'. There are also instances where a compound word contains double letters, such as 'cattail' where the first stem ends with a t and the second stem starts with a t.