Mistake: confusing the verbs "ser" and "estar," which both translate to "to be" in English, but have distinct uses in Portuguese.
Mistake: incorrect placement of object pronouns in relation to verbs.
Mistake: misusing prepositions or omitting them when necessary.
Mistake: incorrect conjugation of irregular verbs.
Mistake: incorrectly forming plurals of nouns and adjectives.
Mistake: failing to match nouns with the appropriate gender or failing to make agreement between nouns and adjectives.
Mistake: omitting or misplacing accent marks.