the tall boy with blond hair who is locking his bicycle 是整句话的主语is是系动词 from finland作表语
其中整句话的主语包含由who引导的定语从句,先行词是the tall boy with blond hair
而定语从句的主语是who,谓语是 is locking （现在正在进行时）,宾语是his bicycle
结构清楚了,意思就出来了：正在锁自行车的那个金发高个男孩,来自芬兰.