Learn Polish: Tomaszu, to jest nasz czternasty pracownik. - Tomasz, this is our fourteenth employee.

It comes from the regular number czternaście (14). Ordinal numbers behave just like adjectives, so they must match the noun they describe. Because pracownik is a masculine noun in the nominative case, the ordinal number drops the -ie and takes the masculine ending -y to become czternasty.

Why is the name spelled Tomaszu instead of Tomasz?

Tomaszu is in the vocative case, which Polish uses when addressing someone directly, like calling out their name. For masculine names ending in a sound like sz, cz, or rz, the vocative ending is typically -u.

Doesn't the verb jest usually require the instrumental case? Why is it pracownik instead of pracownikiem?

You are right that describing someone's profession usually takes the instrumental case, for example: On jest pracownikiem (He is an employee). However, there is a special rule for the phrase to jest (this is). When you use to jest to point someone or something out, the noun that follows always stays in the basic nominative form.

What would this sentence look like if we were pointing out a female employee?