Your English is perfect.
Yes the statement is right.
Dy/dx = should me slope in this case.
A slope of value zero is a horizontal line on the x and y coordinates.
A slope of value infinity is a vertical line on the x and y coordinates.
Since for an infinity, the line is vertical line on say a graph sheet.
So a vertical line drawn on any graph sheet would be parallel to your y axis on the graph sheet.
I hope this helps.
If it doesn't just let me know.
