they,reach,a decision.主,谓,宾.
句子成分有 主谓宾,定状补.否定副词放句首,句子要倒装,如：rarely,hardly等含否定含义的词.
Only after后面是一个条件,did起强调作用,说明做出这个决定很不容易.这句句子可以这样看：They reach a decision（ after they had discussed the matter for several hours),括号里是条件状语.谓语是动词,主语是动作的执行者,宾语是动作的承受者.定语起限定作用,状语有时间状语,地点状语,条件状语等,补语是补足说明的.