Learn Spanish (Latin America): Al preparar la caña, hay que usar una buena carnada. - Upon preparing the fishing rod, one must use a good bait.

The structure al + infinitive translates to "upon preparing" or "when preparing." It's a very common, concise way to connect two events happening around the same time. While you could say cuando preparas (when you prepare), al preparar avoids needing to conjugate the verb or pin down a specific subject, making it perfect for general statements like this one.

Could I use tienes que usar instead of hay que usar in this sentence?

Yes, but it changes the tone slightly. The phrase hay que expresses an impersonal obligation, meaning "one must" or "it is necessary to." Because the first half of the sentence (al preparar) doesn't name a specific person, using hay que keeps the whole sentence as a general rule for anyone fishing. If you said tienes que, you would be giving direct advice specifically to the person you are talking to.

Does caña just mean "fishing rod"?

Not exclusively! The word caña translates literally to "cane," "reed," or "stalk" (for example, caña de azúcar is sugarcane). However, in the context of fishing, it's the standard shorthand for caña de pescar (fishing rod).

Why is it una buena carnada instead of una carnada buena?