Learn Portuguese (Portugal): Dizem que o mexerico começou ontem, mas eu lá sei. - They say the piece of gossip started yesterday, but what do I know.

This is the pragmatic focus taught in this lesson. Instead of referring to a physical place, lá acts as an emphatic marker of doubt or ignorance. Saying eu lá sei translates roughly to "what do I know?" or "how should I know?", effectively distancing the speaker from the information.

Could I just say sei lá instead of eu lá sei?

Yes, sei lá is a very common conversational expression in Portugal meaning "I have no idea" or "beats me." In this specific sentence, however, the speaker is contrasting their own lack of knowledge with what others are saying, so including the subject pronoun eu makes eu lá sei fit the contrast perfectly.

Who is the subject of Dizem at the beginning of the sentence?

There is no specific subject here. Using a verb in the third-person plural without a pronoun is the standard Portuguese way to express the impersonal "they say," "people say," or "rumor has it."

Why is começou in the Pretérito Perfeito (simple past) rather than the Imperfeito (imperfect)?

Although the spreading of the mexerico (gossip) might feel like an ongoing situation, the actual event of it starting was a single, completed action tied to a specific time marker: ontem (yesterday). Therefore, the simple past is the required tense.