Learn Polish: Nadawca zna wagę dopiero teraz. - The sender only knows the weight now.

In Polish, dopiero is used for 'only' when dealing with time or sequences, implying 'not until this moment' or 'as late as now.' If you used tylko, it would sound like the sender knows 'only the weight' (and nothing else), rather than knowing it 'only now.'

How would the meaning change if we swapped dopiero for już?

The phrase już teraz means 'already now' and suggests the sender found out the weight earlier than expected. By using dopiero teraz, the sentence implies a delay: the sender was supposed to know the weight earlier, but didn't find out until right now.

Why does this sentence use zna instead of wie for 'knows'?

You use znać (here as zna) when you know a specific noun, like a person, a place, or a concept like wagę (the weight). You would use wiedzieć (which becomes wie) if you were describing a fact using a full clause, such as 'The sender knows how much the package weighs.'

I know waga means weight, so why is it wagę in this sentence?

Because it is the direct object of the verb zna. Feminine nouns ending in -a like waga change their ending to -ę in the accusative case, which is required here since the weight is the thing being known by the sender.