Answer:
"No one answered the ad but me": Here, but is used as a preposition. When “but” is used before an object pronoun, it means that only that person did the action. In this case, only I answered. It’s used to mean only.
"There is but one choice to make in this case": “but” is used as an adverb. It also means only or just, for this case, it means that there is only one choice to make.
"Sadie said she would go but she had to bring her sister": in this case, the word "but" is used as a conjunction. Here “but” is being used to connect two ideas or clauses that contrast at the same time. In the sentence, the 1st idea (Sadie said she would go) connects and, at the same time, is contrasting the 2nd idea (She had to bring her sister).
Hope this helps! :)
