From then on; became funny and cheerful
句意:他去年开始学相声。从那时起,他变得有趣和开朗。
根据last year(去年)可知本句时态为一般过去时。
根据crosstalk (相声)可知本题应选bocame funny and cheerful(变得有趣和开朗),而become的过去式为became。
分析句子结构,第一空位于主语he(他)之前,且用逗号隔开,应该选择时间状语from then on(从那时起),位于句首首字母大写,故填From then on;第二空位于主语之后,应该是句子的谓语部分,故填became funny and cheerful
故答案为:
From then on; became funny and cheerful