- The longer a word has been in the language, the less likely there is to be a word-break. 'Blackbird'. for example is one word. Obviously this started life as two words. (In fact a female blackbird is not a black bird; it is brown.) But in order to apply this 'rule' you would have to use a dictionary anyway; which makes it useless for language students.
- In attributive adjectives, a hyphen helps (but not everyone cares). For example, a high-level grill is not both high and level.
(Long time no see )