The assumption Portuguese speakers can speak Spanish, but Spanish speakers cannot speak Portuguese without formal study rings as a half-truth. Portuguese retained more of Latin than the Spanish language. To the untrained ear, what may sound like Spanish is really an understandable muddle.
Consonants, Vowels, and Dipthongs
With regard to speaking Portuguese and Spanish, it seems the provinces that now comprise Spain placed speaking emphasis on consonants, and those that now comprise Portugal placed speaking emphasis on vowels and diphthongs. In Portuguese we have coração and its plural form corações with the diphthongs ão and ões, whereas in Spanish those endings are ón and ones. In Portuguese we also have au and its plural form aus in pau and paus, and in Spanish lo and its plural form los in palo and palos.
Accent Marks (Diacritics)
Then we have different accent marks (diacritics) from Portuguese to Spanish. For example, ñ does not exist in Portuguese. However, a very similar sound is written as nh. The ll in Spanish is similar to the lh in Portuguese. Ç and ç (c cedilla) are not used in Spanish, as well as the grave accent mark ` and the circumflex ^ .
False Cognates, Better Known as Treacherous Little Words
There is also the question of false cognates, those treacherous words that originate from Latin, but have changed in meaning from one language to another. A good example of this is the word the Portuguese word esquisito. In Spanish this looks like it corresponds to the Spanish exquisito. Wrong! Wrong! Wrong! From Portuguese to Spanish this word is a “false friend". In Portuguese it means weird and the Spanish corresponds in meaning to the English as exquisite. You can see why these treacherous words are called “false friends" and how they can get you into trouble very easily.
Subjuntives and Infinitives
The subjunctive mode exists in the two languages. However, the future subjunctive in Spanish is used almost exclusively in legal documents, whereas, in Portuguese, the future subjunctive is used in everyday speech. There is also the use of the personal infinitive, which does not even exist in Spanish. The Portuguese personal infinitive is a way out of having to use the present subjunctive with the use of para in the personal infinitive clause and para que with the present subjunctive. Use of the Personal Infinitive: Para eu fazer isto and use of the Present Subjunctive: Para que eu faça isto. The advantage of the personal infinitive is that the verb conjugation is easier to form than that of the present subjunctive. For those speakers who are weak in the Present Subjunctive in Portuguese, they can still be functional without having to strain the brain, by implementing the Personal Infinitive instead.