"He was pretending to be a vampire as a prank but his fangs fell out."
- "He was pretending to be a vampire as a prank but his fangs fell out."
"he was pretending to ~" 「彼は〜のふりをした」
"pretending to be a vampire" 「吸血鬼のふりをした」
"as a prank" 「悪ふざけで」
"but" 「が」
"his fangs" 「偽の牙」
"fell out" 「取れてしまった」