"How do you know this?"

Translation: Comment sais-tu ça ?

[Dugan4]

Why not "Comment ca sais-tu"?

[georgeoftruth]

The object pronoun "ça" does not go before the verb it's the object of.

[briantreiser]

Could you use "cela" instead of "ca?" If so, is there a difference in nuance?

[georgeoftruth]

Yes, you could. But keep in mind that generally, "cela" is "that" and "ceci" is "this". "Ça" is the informal version of "cela" and "ceci". The latter two are rarely used in spoken French.

[Lillian667151]

How would you say it using the formal Vous.

[georgeoftruth]

• Comment savez-vous ça ?

[BfrUfHp7]

Why doesn't it accept comment savez-vous ca?

[georgeoftruth]

"Comment savez-vous ça ?" is fine and accepted.