Learn Spanish (Spain): Deberías usar una tirita para ese dedo. - You should use a plaster for that finger.

Deberías is the conditional tense of deber (to have to / must). While debes strongly means 'you must', using the conditional softens it to 'you should', which is the standard and polite way to give advice in Spanish.

Is the word tirita used in all Spanish-speaking countries?

No, tirita is specifically used in Spain for a plaster or Band-Aid. If you travel to Latin America, you will usually hear people call it a curita instead.

Why do we use para instead of por before ese dedo?

We use para here because it shows the recipient or the intended destination of the object. The plaster is meant for the finger. You can usually rely on para when talking about who or what receives an item.

Why use ese instead of aquel for 'that' finger?

Ese is the demonstrative adjective used for things close to the person you are speaking to. Since the finger belongs to the listener, it is physically right there with them, making ese the perfect choice over aquel (which implies something far away from both of you).