Learn Polish: Jestem teraz w pracy.

In Polish, prepositions do not always match English perfectly. To say 'at work' or 'at school', Polish uses w (which normally means 'in'). So you literally say 'in work' (w pracy) and 'in school' (w szkole).

What is the rule for using w versus na for locations in general?

Generally, w means you are physically inside a building or enclosed space, while na is used for open spaces, events, or surfaces. However, some common places are fixed exceptions you just have to memorize. Praca (work) always takes w, whereas a word like uniwersytet (university) always takes na.

The dictionary word for work is praca. Why does it change to pracy here?

Because it comes after the preposition w to express a location. This triggers the Locative case. For feminine nouns ending in -ca like praca, the final -a changes to -y in this case.

I do not see the word for 'I' here. How do we know who is at work?

The verb jestem already means 'I am'. In Polish, you usually drop subject pronouns like ja (I) because the verb's ending tells you exactly who is doing the action. Adding ja here would actually sound unnatural unless you wanted to strongly emphasize that YOU are at work, not someone else.