It might be helpful to review the difference between subject and object pronouns in English (a classic ACT/SAT topic) before jumping into the differences between Spanish and Portuguese. Notice, however, the surface level similarities between the cousin languages.
Subject pronouns are straightforward: we use them to replace nouns that are in subject position, meaning that they come before a verb.
Object pronouns are a bit more complicated. In Spanish, there are three different types of object pronouns: direct, indirect, and prepositional.
Direct Object Pronouns
These replace nouns that are being directly acted upon. For instance:
I met her at the beach yesterday.
Eu a conheci na praia ontem.
(Colloquial Portuguese: Eu conheci ela na praia ontem.)
Yo la conocí en la playa ayer.
Indirect object pronouns replace nouns (usually people) that are not being acted upon directly, but are receiving the direct object of the sentence:
I give him homework after every class.
Eu lhe dou liçāo de casa depois de cada aula.
(Colloquial Portuguese: Eu dou liçāo de casa para ele depois de cada aula)
Yo le doy tarea después de cada clase.
Here, homework is the direct object -- the thing being given -- while him is the person being given that item, hence the indirect object.
Notice that in Portuguese, the difference between the direct and indirect object pronouns is often ignored in everyday speech; Brazilians use subject pronouns no matter the noun’s position vis-à-vis the verb, and they often make use prepositional phrases to avoid using the object pronoun, as shown above. Spanish behaves more like English or French in its retention of the distinction between subject pronouns, direct/indirect object pronouns, and even prepositional pronouns:
This is very hard for you.
Isto é muito difícil para você.
Esto es muy difícil para tí
Moreover, be sure to not forget that genitive pronouns (possessive pronouns) exist in Spanish, as in English and French! In Portuguese, the genitive form is constructed by adding a definite article to the possessive adjective, whereas Spanish has a different form. This distinction is an important one:
This test is mine.
Essa prova é a minha.
Esa prueba es mía.
Essential Logical Connectors
A lot of prepositions, introductory phrases, and logical connectors in Spanish will be familiar to speakers of Portuguese. Some, however, are quite different. Be sure to know the ones that differ most from Portuguese:
Luego – don’t confuse this one with logo! “Luego” in Spanish means “then” or “next,” not necessarily “quickly” or “very soon” as in Portuguese.
ES — PT
Todavía – ainda
Sin embargo – porém
Aunque – embora
Hace falta – é preciso
Ademas — além disso
Pero — mas
Así que — assim que, entāo
Entonces — então, pois
In general, Spanish and Portuguese verbs behave relatively similarly in their written form. One major difference lies in the subjunctive. For one, Spanish does not have a future subjunctive; in cases where the future subjunctive is used in Portuguese, hispanohablantes will usually substitute the present or imperfect subjunctive. Sometimes, Spanish speakers forgo the subjunctive altogether, especially when talking about hypotheticals or plans for the future:
“Se for para o Brasil, você precisa visitar o Rio de Janeiro.” (future subjunctive)
“Si vas a Brasil, tienes que visitar Rio de Janeiro” (present indicative)
Overall, however, speakers of Portuguese should be relatively familiar with Spanish verbs. That said, Spanish has a lot of "stem-changing" verbs that complicate mutual intelligibility, particularly in the subjunctive. These verbs behave a bit erratically, changing in spelling and thus deviating significantly from the Portuguese equivalent.
Make sure you know the meaning of these twelve essential irregular verbs. Don’t forget how to conjugate them in the present and imperfect* subjunctive tenses:
Infinitive / present subjunctive (3rd person sing) / imperfect subjunctive (3rd person sing)
Estar / esté / estuviera
Ser / sea / fuera
Haber / haya / hubiera
Tener / tenga / tuviera
Hacer / haga / hiciera
Poder / pueda / pudiera
Conocer / conozca / conociera
Saber / sepa / supiera
Venir / venga / viniera
Pedir / pida / pidiera
Querer / quiera / quisiera
Decir / diga / dijera
Note: don’t be overwhelmed(!), but the imperfect subjunctive in Spanish actually has two forms. The one listed above is more common, but you might see the other form on the SAT. All you do is swap out the -era ending for -ese: estuviese, fuese, hubiese, etc.
While context clues and shared Latin roots will help a lusophone tremendously on the Spanish SAT Subject Test, there are lots of false cognates between the two languages. Here are the eight words most likely to trip you up on the SAT Subject Test:
BR-PT: to take away, to steal, or to obtain (as in tirar a 36 on the ACT!)
ES: to throw
(quitar is the Spanish verb for “to take away” – but you would not say “quité un 36 en el ACT;” instead, try the verb sacar)
BR-PT: ready, done, finished (ES: listo/a)
ES: fast, quickly, right away
BR-PT: to fix, to mend, to repair (ES: arreglar)
ES: to schedule
BR-PT: to wake up (ES: despertarse)
ES: to remember
BR-PT: folder (ES: carpeta)
ES: pasta (e.g. al dente, primavera, with pesto, etc.)
BR-PT: farm, ranch (ES: granja or finca)
ES: site, place
BR-PT: scene (ES: escena)
BR-PT: to develop (ES: desarrollar), to involve (ES: involucrar)
ES: to (un)wrap (e.g. a package)