"It is only his imagination."
Translation:C'est seulement son imagination.
"imagination is masculine, not feminine"
No, "imagination" is feminine. The reason you need to use "son" here is so that it can be properly pronounced (as opposed to saying "sa imagination", two vowels after each other).
See http://french.about.com/od/grammar/a/adjectives_possessive_2.htm : "When a feminine noun begins with a vowel, the masculine possessive adjective is used".
Me too and on this very question as well. Here is what I found via Google: http://french.about.com/library/weekly/aa032500.htm http://www.frenchtoday.com/blog/cest-versus-il-elle-est
That helps a lot but try as I might I can't put together why it applies to this situation. "Only" is an adverb so it doesn't count and I guess "his" is a determiner so because the determiner came next instead of an adjective, we use Ce instead of Il.