Learn Portuguese (Portugal): Antes de entrar num prédio inexplorado, certifico-me de que não há perigo. - Before entering an unexplored building, I make sure that there is no danger.

The reflexive verb certificar-se requires the preposition de to connect to its object. In Portuguese, when a verb naturally governs a preposition, you must retain that preposition even when introducing a clause with que. While English drops the preposition to say make sure that, Portuguese strictly requires de que.

Why does certifico-me de que trigger the indicative verb há instead of the subjunctive haja?

In this context, certificar-se means to verify or confirm an existing fact. Because you are confirming a reality (that there is no danger), the clause takes the indicative mood. If the verb were used to express influence or an action to be done, such as ensuring that someone else completes a task, it would trigger the subjunctive.

Why is the infinitive entrar used after Antes de rather than a conjugated verb?

When the subject of the subordinate clause is the same as the main clause (I am the one entering, and I am the one checking), Portuguese uses the infinitive after prepositional phrases like antes de. There is no need for a conjugated verb or even the personal infinitive here, because the subject is perfectly clear from certifico-me.

Is prédio always a tall apartment building?