The assignment was originally due on Tuesday, but the instructor changed the date to Thursday.
Here's why:
* "However" is a conjunctive adverb that signals a contrast. It's best used when the two sentences have opposing ideas.
* "But" is a coordinating conjunction that also indicates a contrast. It's a simpler and more direct way to show the change in the due date.
* A comma is needed after "Tuesday" to separate the two clauses.
The combined sentence clearly shows the original due date and the change made by the instructor.