形容词后的to都是介词吗?

不是的.
to有两种情况,含有to的短语里面：
1.是固定搭配,既不能用其他的短语代替想表达的意思的：
look forward to(prep.)+doing something期待某事
set down to doing something开始着手某事
2.可以替代的：
want to do something=would like to do something
have to do something与need to do something意思相近.
to是介词情况的短语还是需要靠记忆.