Learn Polish: Telefon jest na stole, a gazeta pod stołem. - The phone is on the table, and the newspaper under the table.

This happens because Polish changes the endings of nouns depending on the prepositions before them. The preposition na (on) requires the Locative case, which turns stół into stole. The preposition pod (under) requires the Instrumental case, turning it into stołem.

Where did the letter ó from stół go?

It is very common in Polish for the vowel ó in a base word to change to a regular o when you add grammatical endings. In stole, the consonant ł also changes to l. You will see these kinds of letter swaps often as words change form.

Why use a to mean "and" instead of the usual word i?

While i simply joins things together, a is used when there is a slight contrast or a shift in focus. Because you are pointing out two different objects in two different locations, a acts like "and" or "while" to highlight that comparison.

Shouldn't it say a gazeta jest pod stołem?

You can include jest (is), but it is perfectly fine to leave it out! Just like in English, if the verb is exactly the same in both halves of the sentence, Polish speakers often drop the second one to avoid repeating themselves.