"He was a grave and secretive man."
Translation:C'était un homme grave et secret.
"Il est" followed by a modified noun has to change to "c'est".
A modified noun is a noun preceded by a determiner: article, possessive adjective, demonstrative adjective, number, indefinite adjective.
The noun is "homme" and the modifier is the indefinite article "un".
- He is a... man = C'est un homme....
The same rule applies to "elle est" and to "ils/elles sont" which change to "ce sont".