After the/a/an/ some/any/many / his etc / you use a noun. Check if there is not a noun after the gap. If there is, use an adjective.
His ... to succeed impressed us very much. DETERMINE answer: determination
If there is a gap in front of a noun or after a form of to be you get an adjective.
It is ... to see how far he can jump. ASTONISH answer: astonishing.
He received a very ...... letter. BEAUTY anser: beautiful
If there is a gap after a verb or before an adjective you use an adverb.
They spoke .... about the difficult situation. INTELLIGENCE answer : intelligently
Nobody knew where the .... difficult test would take place. EXTREME answer: extremely
Sometimes you need to add a prefix or suffix as well.
After the divorce Fred felt very ... for quite a long time. HAPPINESS answer unhappy