A score asks for C-sharp. Another asks for D-flat. At a conventionally tuned piano, your finger can land on the same black key in both cases. The two written names are not a printing mistake: they describe the pitch through different letter names and alterations.
The University of Puget Sound’s explanation of enharmonic notes shows how notes that sound the same can be written differently on the staff. The distinction becomes easier to grasp when you look at the keyboard and notation together.
Find C, then move one semitone higher to C-sharp. Next find D and move one semitone lower to D-flat. Compare the destination. Repeat slowly with F-sharp and G-flat, naming the starting letter as well as the alteration.
Do the exercise without a backing track first. Its purpose is to connect a written instruction with a location, not to test speed. If a keyboard app labels only one name, write the alternative in your own practice notes.
Look at a D-major scale: D, E, F-sharp, G, A, B, C-sharp, D. Each successive scale degree has its own letter name. Replacing F-sharp with G-flat would obscure that pattern, even though the piano key stays the same.
Try writing the scale in a notebook and circling the letter names before marking the sharps. This makes spelling something you can inspect rather than memorise as an unexplained exception.
Enharmonic names are not limited to black keys. The cited lesson also gives white-key examples, including B-sharp and C. Learn these in small groups instead of relabelling an entire score at once.
Keep the scope of the demonstration clear: a piano provides fixed keys. Questions about expressive tuning on other instruments require additional context. For this exercise, the useful discovery is that one keyboard location can have two written identities, and the surrounding music helps explain the choice.