I am tutoring another US college student in a Calculus 1 class. Initially, she was having trouble with basic concepts, but after much prodding most of the conceptual difficulties seem to have been alleviated. However there is one persistent problem I've never encountered with another student: after integrating a function, she'll remove the $\int$ but leave $dx$ (or $du$ etc.). For example, she will do this:
$$\int 2x\, dx = x^2 + C\, dx$$
The $dx$ will hang around in her later calculations, and she'll express that something seems wrong (especially when none of the multiple-choice answers match - we're working with old exams), but I'll need to explicitly point out what happened before she fixes it. If I say "Why is the $dx$ still there?" she'll immediately realize that it was a mistake, though sometimes she will still ask why keeping it is wrong.
My first attempt at explaining the problem was "the $dx$ goes poof when the integral goes away" (this being how I learned it). My next was "the $dx$ marks the other end of what's being integrated, it's a boundary along with the $\int$". My third, after looking up what the $dx$ represented and finding What is $dx$ in integration?, was "it's like the $\Delta x$ in a Riemann sum, and when you evaluate the limit there is no $\Delta x$ still hanging around". All of these have been used more than once. Each attempt has stopped the forgetting for some more time but then she gets into the weeds of an integral and leaves the $dx$ on again while wrapped up thinking about something else.
How can I hammer this home so that it sticks? She is most confident in Riemann sums (that's part of why I used them in my third attempt) so an answer which leverages them would be appreciated but not required. We do online tutoring; anything that requires me being physically present will not work.