"We are speaking about sandwiches."

Translation: Mówimy o kanapkach.

[kiddo-depido]

How would you say "about a sandwich"?

[immery]

o kanapce

[mgaristova]

I wrote "rozmawiamy" and it was accepted.

[Vengir]

That should be right. „Mówić” implies more a one way communication, „rozmawiać” is two way.

[mgaristova]

That was really very useful, Vengir, thank you, you deserved a lingot !

[SashaKW]

Why do you use the locative case in this sentence?

[JerryMcCarthy99]

As noted above, "o" takes locative.

[SashaKW]

Thank you!

[Benjy785856]

'Mowimy o kanapkami' was wrong...?

[Jellei]

Yup, it's wrong, wrong case. "kanapkami" is Instrumental. Like "Jesteśmy kanapkami" (We are sandwiches) if we were starring in "Sausage Party".

"o" = "about" takes Locative.