compound nouns and compound adjective

Compound nouns can be formed using the following combinations of words:-

Noun + Noun toothpaste
Adjective + Noun monthly ticket
Verb + Noun swimming pool
Preposition + Noun underground
Noun + Verb haircut
Noun + Preposition hanger on
Adjective + Verb dry-cleaning
Preposition + Verb output

Words like densely-populated are compound adjectives and they are made up of two or more words, normally with hyphens between them. Something that is dense
contains a lot of things or people in a small area. Thus a densely-populated town or city is one with a high population count within the city boundaries. A densely-wooded hill would be one that is difficult to get through because the trees are so close together.
adj / adv + past participle

Adjective or adverb plus past participle is one of the most common patterns for forming compound adjectives. Some common examples would include:

cold-blooded kind-hearted old-fashioned open-minded
brightly-lit deeply-rooted densely-populated well-behaved

Adj / adv / noun + present participle

Here are some common examples:

good-looking hard-wearing free-standing
far-reaching long-lasting never-ending
labour-saving mouth-watering record-breaking