Learn Finnish: Onhan meillä vielä polttopuita. - We still have firewood, you know.

It adds a reassuring or reminding tone, translated here as "you know". It implies that the fact is already known to the listener, as if saying "Don't worry, we do still have firewood, remember?"

Why does the sentence start with the verb Onhan instead of Meillä?

The clitic -han usually attaches to the first word of the clause. By moving the verb on to the front and attaching -han, the speaker emphasizes the existence of the wood. If you started with Meillä, you would say Meillähän on..., which would shift the focus to "We (as opposed to someone else) have firewood, you know."

Why is the verb on singular when polttopuita is plural?

In Finnish possessive structures like meillä on (we have), the verb is always in the third-person singular (on), no matter what is owned. You will never use the plural ovat in this exact structure.

Why is polttopuita in the partitive plural?

Polttopuita is the partitive plural of polttopuu (firewood). The partitive is used here because it refers to an unspecified, uncountable amount of wood, meaning "some firewood" rather than a specific, complete pile.