Learn Polish: Ten chłopiec pracuje w kawiarni. Jest kelnerem. - This boy works in a cafe. He is a waiter.

When we talk about someone's profession using the verb być (to be), we must put the noun into the Instrumental case. The basic word is kelner. Because it is a masculine noun ending in a consonant, we add the ending -em to make it kelnerem.

Why does the second sentence start with "Jest" instead of "On jest" (He is)?

Polish often drops subject pronouns like on (he) because the context and the verb make it clear who we mean. Here, Jest translates to "he is" because we already know we are talking about Ten chłopiec (This boy) from the previous sentence.

The dictionary word for cafe is "kawiarnia". Why does it change to "kawiarni" here?

When saying that something is "in" a location using the preposition w, Polish uses the Locative case. For feminine nouns ending in -nia like kawiarnia, the ending changes to -ni, which gives us w kawiarni.

Why do we use "Ten" for "This" instead of "Ta" or "To"?

The word for "this" has to match the gender of the noun it points to. Because chłopiec (boy) is a masculine noun, we use the masculine form Ten. We would use Ta for feminine nouns and To for neuter nouns.