But or Although?
Exemple : ‘’It was raining, but we decided to go.’’
"But" and "although" have the same purpose but are used in different structures. "But" cannot appear at the beginning of a sentence. Instead it can appear in the middle in order to separate two contrasting clauses.
Exemple : ‘’Although it was raining we decided to go anyway.’’
On the other hand "although" can be used at the beginning of a sentence in order to show contrast.