In English, we often end sentences with prepositions when talking about abstract ideas, like "what the movie is based on" or "what I believe in."
French grammar does not allow prepositions at the end of a sentence. When "what" refers to an abstract idea, French combines ce (that) + a preposition + quoi (which/what).
Ce en quoi
If the verb requires the preposition en (like croire en, to believe in), use ce en quoi (what... in).
| Yet, the future of the universe is what the fan believes in. | Pourtant, le futur de l'univers est ce en quoi le fan croit. |
Ce à quoi
If the verb requires à (like s'attendre à, to expect), use ce à quoi.
| Yet, what I expect is very different. | Pourtant, ce à quoi je m'attends est très différent. |
When the subject is a noun (like le fan), French often inverts the subject and verb after the relative pronoun:
| That is what the science fiction fan expects at the convention. | C'est ce à quoi s'attend le fan de science-fiction à la convention. |
Ce sur quoi & Ce pour quoi
Similarly, verbs requiring sur (like se baser sur, to base oneself on) use ce sur quoi, and sentences with pour use ce pour quoi.
| I would like to know what the theory is based on. | Je voudrais savoir ce sur quoi se base la théorie. |
| The future is what we are working for. | Le futur est ce pour quoi nous travaillons. |
Starting a Sentence
You can also use these phrases at the beginning of a sentence to mean "What [subject] [verb] is...":
| What this script is based on is a secret. | Ce sur quoi ce scénario se base est un secret. |
| What you are expecting is the hologram of the creature. | Ce à quoi tu t'attends, c'est l'hologramme de la créature. |