The mathematical expression of some law or principle of physics is usually stated for the most general situation conceivable. These equations are typically found highlighted in the textbook or in the summary at the end of a chapter. Because they are so general, they are directly applicable to only very simple problems (which pertain to the relevant physics that produced those generic equations).

Real problems almost always involve more complex applications of the generic equations. As a result, the symbolic labels of these generic equations cannot be mapped onto the variables in a one-to-one fashion. This means that you will have to generate your own personal labels for the knowns and unknowns in a problem, and then map them onto the generic equations.

The generic equations are templates that can be applied to different parts of a problem. Each variable in a generic equation is a pointer to a concept. It is not the variable itself -- as a symbol -- that is important, but the concept that it signifies. You could use any symbol to represent the concept. The meaning of any symbol you see is not fixed in stone and can vary from problem to problem. What is fixed is the concept which the symbol tags. To be successful in solving problems, you will need to be consciously aware of the concept to which a symbol points.

"When does Friday come before Thursday this week?"
Answer: When you look them up in a dictionary.
This is an example of a joke that plays on the difference between the symbols "Friday" and "Thursday" which point to days of the week, and the symbols "Friday" and "Thursday" taken as a collection of letters. Taken as a collection of letters alone the answer is perfectly correct, but it is not correct for the meaning of the words/symbols which represent a day of the week.