The practice card generates a fresh problem on demand, filtered to one kind of function at a time. The filtering is the point.
One rule at a time
If you are drilling the chain rule, a random mix of polynomials and logarithms is noise. Choosing Trigonometric or Exponential and log narrows the pool to problems that exercise the rule you are working on, and picking the same category again rolls another one.
Generated, not stored
Each category holds a set of templates with random coefficients rather than a fixed list, so the same category keeps producing new problems. The template shapes are chosen to stay solvable by hand in a minute or two — big enough to need the rule, small enough that the algebra is not the obstacle.
Checked before it is shown
A generated problem is parsed and differentiated before it is displayed. If either step fails, the generator draws again. You never see a problem the engine cannot solve.