Learn Spanish (Latin America): El evento es muy formal; tu ropa informal es un problema. - The event is very formal; your informal clothes are a problem.

In Spanish, ropa is a collective noun and is almost always treated as singular, much like the word 'clothing' in English. Because of this, you must say tu ropa es instead of translating 'clothes are' literally into a plural form.

'Problema' ends in '-a'. Why do we use the masculine 'un' instead of 'una'?

Problema is one of several common Spanish words ending in '-ma' that are masculine because they originate from Greek. Other everyday examples you will encounter include el idioma, el sistema, and el clima.

Why use es instead of está to describe the event, since it only lasts for one night?

We use ser (es) because we are defining the fundamental nature or essential characteristic of the event. Even if the party only lasts a few hours, being formal is a defining trait of this specific event, not a temporary mood or physical state.

Since the current lesson covers exclusion markers, how could I use one to say 'There is no problem except your informal clothes'?

You can use excepto, salvo, or menos directly before the noun phrase. For example, you could say: No hay problema salvo tu ropa informal or Todo está bien menos tu ropa. Notice how the exclusion marker connects directly to the noun without needing extra prepositions.