An independent variable is like a knob you can turn – it’s an adjustable thing. A dependent variable is what changes when you turn the knob. You can change the hours of study, and as a result the course grade changes. That makes the hours of study the independent variable and the course grade the dependent variable.
Can you change the course grade and as a result the hours that were studied change? That’s hard to make a case for, it’s not the simple relationship of hours –> grade.
You feed the cat, it gets fat. Feeding is independent, feline heft is dependent.
You work more overtime, you make more money. Overtime hours are independent, money is dependent.
You change the pH, the enzyme activity changes. You plant more trees, the stream quality gets better (20 years later). You get used to spotting which variable is which.