只有当他亲眼看见那些贴满小区的海报时，才真正相信中国诗词大赛是多么受欢迎．（Only）．

Question

只有当他亲眼看见那些贴满小区的海报时，才真正相信中国诗词大赛是多么受欢迎．（Only）
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Answer 1

Only when he saw the posters which were put up/posted everywhere in the community/neighbourhood with his own eyes did he truly/really believe that the Chinese Poetry Contest was very popular/warmly welcomed．
根据句意“只有当他亲眼看见那些贴满小区的海报时，才真正相信”可知，使用only加状语从句位于句首使用倒装的句式比较合适．which were put up/posted everywhere in the community/neighbourhood为一定语从句，修饰先行词the posters；believe后面的that引导的是一个宾语从句；be very popular/warmly welcome大受欢迎．