Inappropriate and unsuitable are both qualitative adjectives.They describe the quality of something, and are very similar in meaning and usage. They are both placed as modifiers before nouns and they are both used as complements after the verb be, although inappropriate is perhaps more commonly used in this way, especially with the pronoun it.
inappropriate to/for something: Her clothes were inappropriate to/for the occasion
inappropriate (for somebody/something) (to do something) : It would be inappropriate for me to comment.
unsuitable (for somebody/something): He was wearing shoes that were totally unsuitable for climbing.
The clothes she was wearing were quite unsuitable/inappropriate for the cold weather.
They considered him quite unsuitable for their daughter.
These roads are unsuitable for use by heavy vehicles.
Does this dress suit me? No, it doesn’t. And it's very unsuitable/inappropriate for formal occasions.
It is a very violent film and is considered unsuitable/inappropriate for children to watch.
Such small flats are really unsuitable for couples with young children.
It is an unsuitable/inappropriate accommodation.
If something is inadequate, there isn’t enough of it or it is not good enough. Usage of this adjective often denotes quantity instead of quality, whereas inappropriate and unsuitable suggest a qualitative response to something.
inadequate for something :The system is inadequate for the tasks it has to perform.
inadequate to do something :The food supplies are inadequate to meet the needs of the hungry.
The action taken to combat the spread of malaria was quite inadequate.
There were not enough seats for all the guests. The supply of seats was quite inadequate.
The Prime Minister gave an inadequate reply to the journalist's question.
Fontes: BBC Learning, Oxford Advanced Learners Dictionary, Longman Dictionary.