要明确什么是系表结构,表语包括系动词以及后面说明主语名称性质状态等,如Our plan is to keep the affair secret.我们的计划是让这件事成为秘密.主语+系动词（be动词）+动词不定式引导的从句.表语即是 is to keep the affair secret.they is likely to injure themselves.这句话中系表结构是is likely to injure themselves