Multiply vs. Exponent

I’m sure a post already exists about this, but I can’t find it. How in the heck do I keep straight when to multiply vs. raise to an exponent? E.g. when finding number of contracts for synthetic stock I multiplied the risk free rate by time instead of raising it to the time component. This is because I had bond returns stuck in my head which of course require that you multiply instead of use exponents at least for one part of the equation. Driving me crazy trying to keep this straight. Any suggestions? Thanks.