"but", "except", "than" as conjunction and preposition

#1 He did nothing but cry.

#2 He had nothing to eat except drink water.

#3 She did no more than sleep the whole day.

#4 You have no choice but to perform the back test yourself.

#5 I was naked except for my socks.

#6 I want nothing but to save.

#7 It is a fact that most of the people desire to work less but earn more.

We can use "but", "except", "than" both as preposition and "conjunction". I know the difference b/w conjunction and preposition.

But in above sentences i find it difficult for me to decide which one is preposition and conjunction. Words like "than", "but" "except" always confused me whether they are used as preposition or conjunction. I need help in those words if you can explain me.

Thank you