"But" is one of the coordinating conjuctions (For, And, Nor, But, Or, Yet, So) and indicates a contrast between two independent clauses.
Example:
<u><em>"Waited so patiently for our report </em></u><u><em>BUT </em></u><u><em>we have not submitted it yet" </em></u>
IMPORTANT:
An independent clause is one in which a complement is not necessary to make sense. If you take the sentences before and after the conjunctions, they make sense by themselves; there is no need of complement:
<em>- Waited so patiently for our report </em>
<em>- We have not submitted it yet.</em>
