El accidente fue enorme, la sirena debe de ser de la ambulancia.

Questions & Answers about El accidente fue enorme, la sirena debe de ser de la ambulancia.

Why is it debe de ser instead of just debe ser?
Adding de after deber changes the meaning from obligation ('must' as in 'have to') to probability or assumption ('must' as in 'probably is'). So, debe de ser means 'it probably is', whereas debe ser technically means 'it is required to be'. Note that in everyday spoken Spanish, native speakers often drop the de and use debe ser for both, but keeping the de makes it strictly about probability.
Wait, if 'enorme' (huge) is a description, why use the preterite fue instead of the imperfect era?
Learners are often taught that descriptions in the past always use the imperfect. However, because an accident is a single, completed event, using the preterite fue sums it up as a finished occurrence. If you used era, it would sound like you are setting the scene or background for a longer story, rather than just stating a completed fact.
Why use a form of ser (fue) here instead of estar (estuvo)? I thought temporary states use estar.
We use ser to describe events taking place. Since an accident is an event or occurrence rather than a physical object or a person in a temporary state, we must describe it using ser.
Isn't it repetitive to say debe de ser de la with two des so close together?
It might look repetitive to an English speaker, but it is completely natural in Spanish! The first de is part of the grammar structure debe de (expressing assumption), and the second de is the preposition meaning 'of' or 'from', showing that the siren belongs to the ambulance.

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