should do 情态动词

can do = be able to do
will do = be going to do
should do = ought to do
情态动词都是表示建议
从这里发现 to do （不确定的意思）动词不定式
to 就是不确定去做，做不做由你

finish doing
finish 都结束了，所以是肯定做完了，所以不可能加 to do 不确定的含义了。