何时既可以用带to的不定式又可以用不带to不定式

1.当是跟在这些词后面时let make have,和 sound hear这一系列的感官动词
2.当句子前有个不定式 且有介词连接时.
she tole me to be cheerful and (to) look on the bright side.
像表连接的词有and of except but than ,rather than,