Differential equations - how to form them?

Forum,

I seek to gain an understanding of how differential equations are arrived at.

In most cases, intuition, basic knowledge of physics, or computer simulation can tell us that the unknown variable is dependent on one or more variables. How this translates into an equation is not clear to me.

I ask because I have solved hundreds of ODE/PDEs, but given a sample real-world situation, how would I even formulate the problem, before solving it, seems to be the challenge.

Would appreciate advise.

Best,

wirefree