Question

what is the adjective form of youth and strength

Answer 1

the adjective form of youth is youthful and of strength is strong ...

Answer 2

In grammar, an adjective is 'a word that describes the noun'.  

Examples include -

'The girl next to the tree is tall'

'This cat under the sofa is evil'

'The boys in the game are strong'

'People are so kind'

'This school is strict when it comes to punctuality'

In the given words, 'youth' will change to 'youthful'

Example - 'More than half of India's population is youthful'

and 'strength' will change to 'strong'

Example - 'Men who work in mining are so strong'.