"Hay que" and "se debe" are both used to express obligation in Spanish, but they have slightly different nuances and uses.

"Hay que" is used to express a general obligation or necessity without specifying who must perform the action. For example, in the sentence "Hay que recoger cuando ellos terminen" ('We have to tidy up when they finish'), it implies that it is necessary for someone (in general) to tidy up when they finish, without specifying who exactly.

On the other hand, "se debe" is used to indicate a more formal or specific obligation, often implying a rule, recommendation, or moral duty. It is typically used in more formal contexts and may suggest that a particular person or group should perform the action. For example, "Se debe recoger cuando ellos terminen" ('One must tidy up when they finish') suggests that it is a rule or recommendation that tidying up should occur when they finish. It can imply a sense of duty or expectation directed at a specific person or group.

In summary, "hay que" is used for general, non-specific obligations, such as "Hay que estudiar para aprender" ('One must study to learn'), while "se debe" is used for more formal, specific obligations or recommendations, such as "Se debe respetar las señales de tráfico" ('Traffic signs must be respected'). Understanding these differences helps you choose the appropriate expression based on the context and level of formality required.