Where fluent Spanish speakers start making mistakes in Portuguese, and why: the personal infinitive, a construction Spanish never built.
Spanish speakers who read Portuguese with confidence — who can follow a news article, order in a restaurant, follow a conversation — tend to hit the same invisible wall. It is not vocabulary. It is not the nasal vowels. It is a grammatical structure that simply does not exist in Spanish: the infinitivo pessoal, or personal infinitive.
In Spanish, the infinitive is invariant. Para hablar bien means "in order to speak well," full stop, whoever is doing the speaking. Portuguese works differently. Its infinitive inflects for person: para falar (for one to speak), para falares (for you to speak), para falarmos (for us to speak). Same base verb, different endings, different grammatical subject — and no Spanish equivalent at any register.
The practical trap is this: a Spanish speaker reading Portuguese prose sees an infinitive and automatically reads it as subjectless, the way a Spanish infinitive always is. But a sentence like É importante fazeres isso antes de saíres — "It is important for you to do that before you leave" — contains two inflected infinitives that silently carry their own subjects. Read them as neutral Spanish-style infinitives and the sentence becomes not wrong exactly, but blurred: the who is lost.
The effect compounds in longer sentences, where the inflected infinitive can substitute for a subjunctive clause. Portuguese writers use it precisely because it avoids subjunctive concord while still marking the actor. The Spanish reader, expecting a subjunctive where they know to slow down, instead finds an infinitive and speeds up — at the exact moment the grammar is doing its most work.
One missing construction; a remarkably consistent gap.