Learn Portuguese (Portugal): Só eu arrumei o sótão.

The focus adverb só (only) highlights the word or phrase immediately after it. By placing it before eu, the sentence strictly means "Only I" (and nobody else) did the tidying. If you moved it to say Eu só arrumei o sótão, the focus shifts to the verb or object, meaning "I only tidied the attic" (and didn't clean the basement, for example).

Portuguese often drops subject pronouns. Can I drop eu and just say Só arrumei o sótão?

No, you must keep eu in this specific case. Because só modifies the word directly after it, dropping the pronoun means só would attach to the verb arrumei. The sentence Só arrumei o sótão changes the meaning entirely to "I only tidied the attic" (meaning you didn't do any other chores), rather than saying you were the only person working.

Could I use apenas in this sentence instead?

Yes, Apenas eu arrumei o sótão works perfectly. Apenas is a slightly more formal synonym for só, and it follows the exact same syntactic placement rules to define its scope.

Why use the completed past tense (arrumei) rather than the imperfect (arrumava) here?