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'Entender mal', 'avaliar mal': how Portuguese says 'mis-'

English has a handy prefix for mistakes of understanding and judgment: you misunderstand, misjudge, miscalculate, misdiagnose. Portuguese has no matching prefix for these verbs. Instead, it takes the ordinary verb and adds the adverb mal (badly) after it: entender mal, julgar mal, calcular mal, diagnosticar mal.

English speakers often go looking for a single word that does not exist, or they try to invent one. The Portuguese solution is simpler. If you know the basic verb, you already know the mis- version: just add mal.

Word list
entender malB2 to misunderstand entendi mal a pergunta = I misunderstood the question interpretar malB2 to misunderstand interpretar mal = to misunderstand, to misinterpret avaliar malC1 to misjudge avaliar mal = to misjudge calcular malC1 to misjudge calcular mal = to misjudge julgar malC1 to misjudge te julguei mal = I misjudged you diagnosticar malC2 to misdiagnose diagnosticar mal = to misdiagnose
How it works

The construction is verb + mal. The verb conjugates as usual, and mal never changes. It stays the same for every person, tense, gender and number. It normally comes right after the conjugated verb and before the object.

Acho que você entendeu mal o que eu disse.
(I think you misunderstood what I said.)

With an object pronoun, the pronoun goes in its usual place and mal still follows the verb: você me entendeu mal (you misunderstood me). In compound tenses, mal comes after the participle: eu tinha entendido mal (I had misunderstood).

Rule of thumb: when the English mis- means that someone understood, judged or estimated something incorrectly, translate the base verb and add mal.

The words
Understanding and interpreting

Entender mal is the everyday choice. Interpretar mal is a little more precise and suits texts, intentions and data.

Entendi mal a pergunta e respondi outra coisa.
(I misunderstood the question and answered something else.)
Os jornalistas interpretaram mal as palavras do ministro.
(The journalists misinterpreted the minister's words.)
Judging and estimating

All three of these can translate as to misjudge, but they differ in focus. Avaliar mal means assessing a situation, risk or quality badly. Calcular mal covers numbers, distances and timing. Julgar mal is typically about people and their character or motives.

A equipe avaliou mal os riscos do projeto.
(The team misjudged the risks of the project.)
Eu calculei mal o tempo e perdi o ônibus.
(I misjudged the time and missed the bus.)
Desculpa, eu te julguei mal. Você é uma pessoa muito legal.
(Sorry, I misjudged you. You're a really nice person.)
Diagnosing

Diagnosticar mal works exactly the same way in a medical context.

O primeiro médico diagnosticou mal a doença.
(The first doctor misdiagnosed the illness.)
Watch out
Mal, not mau

Mal is the adverb (badly). Mau and má are adjectives (bad) and describe nouns. After a verb you always need mal, so ela entendeu mau is wrong.

Position changes the meaning

Placed before a verb, mal usually means hardly or barely. Compare these two:

Eu mal entendi a explicação.
(I barely understood the explanation.)
Eu entendi mal a explicação.
(I misunderstood the explanation.)
Words that did fuse

A few words have joined mal into a single form. The noun mal-entendido (a misunderstanding) is hyphenated, while the verb phrase entendido mal is not. Maltratar (to mistreat) is written as one word. These are fixed items, so learn them individually. They do not give you a model for writing new combinations.

Alternatives

In Brazilian speech, errado is very common as an alternative: entendi errado. In formal or technical writing you may see diagnosticar erroneamente.

Not every mis-

The pattern covers understanding and judgment, not the whole English prefix. To mislead is enganar, to misspell is escrever errado, and to misinform is desinformar, which does use a prefix.

Summary

When English uses mis- for getting something wrong in your head (misunderstand, misinterpret, misjudge, miscalculate, misdiagnose), Portuguese uses the plain verb followed by the unchanging adverb mal: entendi mal, avaliou mal, julguei mal. Put mal after the verb, because before the verb it means barely. Use mal here, never the adjective mau.