"Take this off" (i. e. "I want to see you naked") = "Sundej si to".
"Take this off" (i. e. "You look like a scarecrow in it") = "Sundej to".
"Condom" = "preservativ" (the word "kondom" is used in Czech too, but is much less common).
"Mě" is mostly pronounced like "mnye" (with "m" followed by a sound similar to "n"-sound in British "new"). The pronunciation "mye" ("m" followed by a sound like in "yet" in English) is older but very rare nowadays, although it is considered as correct according to the Czech orthoepic norm.