Learn Hardcore Portuguese (Portugal): Ela limpa o espelho com cuidado.

Limpa is the third-person singular present indicative of limpar (“to clean”). So Ela limpa literally means “she cleans” or “she is cleaning.”

Why does the sentence use o espelho instead of just espelho?

Portuguese generally requires a definite article before specific nouns: o espelho = “the mirror.” In English you might drop “the,” but in Portuguese you normally keep it when referring to a particular object.

Why isn’t there any preposition between limpa and o espelho?

Limpar is a transitive verb that takes a direct object without a preposition. You say “limpar algo” = “to clean something,” so you don’t insert a preposition.

How can I replace o espelho with an object pronoun?

The masculine singular direct-object pronoun is o. In European Portuguese you attach it to the verb:
Ela limpa-o com cuidado.
Informally or in speech you might also hear:
Ela o limpa com cuidado.

What does com cuidado literally mean, and why use that instead of an –mente adverb?

Com cuidado literally means “with care.” It’s a noun (cuidado) preceded by com (“with”), forming an adverbial phrase. You could also use the adverb cuidadosamente (“carefully”):
Ela limpa o espelho cuidadosamente.
Both forms are fine; com cuidado is very common in European Portuguese.

Can I move com cuidado to a different place in the sentence?

The most natural positions are right after the verb or after the object:
Ela limpa o espelho com cuidado.
Ela limpa cuidadosamente o espelho.
Putting it at the very beginning (e.g. Com cuidado, ela limpa o espelho) is possible but gives extra emphasis on the manner.

What gender and number are espelho and cuidado, and how do they affect the articles?

Both espelho (“mirror”) and cuidado (“care”) are masculine singular nouns, so they take the masculine singular article o.

Could cuidado ever be plural in this context?

No. Here cuidado is an uncountable noun (“care”). The plural cuidados means “concerns” or “precautions,” which isn’t appropriate for “cleaning carefully.”

Why aren’t there any accent marks in Ela limpa o espelho com cuidado?

Portuguese stress rules place the emphasis on the penultimate syllable when a word ends in a vowel (lim-PA, es-PE-lho, cui-DA-do). None of these need an accent under standard orthography.

How would I express “she is cleaning the mirror carefully” in the European Portuguese continuous aspect?

Use estar a + infinitive:
Ela está a limpar o espelho com cuidado.