Learn Spanish (Latin America): Tienes que usar un pañuelo para estornudar.

In Spanish, it is very common to use para followed by an infinitive verb to express purpose or what an object is used for. So para estornudar translates literally to 'in order to sneeze' or 'for sneezing', which is how Spanish speakers naturally express 'when you sneeze' in this context.

This lesson is about the subjunctive. Why is there no subjunctive used in this sentence?

There is no trigger for the subjunctive here. The phrase Tienes que expresses a direct obligation (you have to), not a wish, hope, or doubt. If you wanted to express a wish using the current lesson's grammar, you would say something like Ojalá que uses un pañuelo (I hope you use a tissue), which does trigger the subjunctive.

Why do we use para instead of por before estornudar?

We use para here because it indicates purpose or the intended use of the tissue ('in order to' or 'for the purpose of'). Por is generally used for the cause or reason behind an action ('because of'). Since the tissue is a tool meant for sneezing into, para is the correct choice.

Could I say Debes usar instead of Tienes que usar?

Yes, absolutely! Debes usar un pañuelo means 'You must use a tissue' or 'You should use a tissue.' However, tienes que usar (you have to use) is slightly more common in everyday spoken Latin American Spanish for expressing a strong practical need or rule.