Imagine you’re listening to a song (une chanson in French or, informally, une toune), and you want to tell your friend that it’s good.
Which one will you say?
• C’est bon, or
• C’est bonne.
Whether you’ve got the word une chanson or une toune in mind, the French word for song is feminine. So it must be c’est bonne, right?
Except… it’s not.
C’est bonne is never used in French. Never. Jamais. As in never-ever-ever with a cherry on top. None of these are ever used either: c’est chaude, c’est froide, c’est mauvaise, c’est grande, c’est petite, c’est belle. Nope!
After c’est, it’s always the unmarked (i.e., masculine) form that’s used: c’est chaud, c’est froid, c’est mauvais, c’est grand, c’est petit, c’est beau. Always. Even when referring to something whose name is a feminine noun in French.
If you’re talking about hot soup (soupe is feminine), you’ll say c’est chaud. If you’re talking about a little cup (tasse is feminine), you’ll say c’est petit. If you’re talking about cold water (eau is feminine), you’ll say c’est froid.
Look now at this example using the feminine toune:
C’est-tu bon, c’te toune-là?
Is that song good?
(literally, is it good, that song?)
C’te is a contraction of cette. It sounds like te with an s sound on the front of it.
As for c’est-tu, it means the same thing as est-ce que c’est. C’est-tu is heard very frequently in spoken language. The tu of c’est-tu only serves to turn c’est into a yes-no question, like est-ce que does. This tu doesn’t mean you. The unmarked form of the adjective is also used after c’est-tu. So you ask c’est-tu bon?, and never c’est-tu bonne?
The same applies to c’était, of course. You don’t say c’était bonne; you say c’était bon, even when referring to something whose name is a feminine noun in French.
Update:
As Cheyne mentioned in the comments, the reason the masculine form is used is because the adjective is agreeing with its subject, which is ce. (C’est is a contraction of ce est.)
