Learn Polish: On nie ma telewizora w pokoju. - He does not have a TV in the room.

Because of the negative word nie. The verb to have (mieć) usually takes the Accusative case for its object, like On ma telewizor (He has a TV). But when you make the sentence negative, the object must switch to the Genitive case. That is why we add -a to make it telewizora.

How do I know to add -a instead of -u to telewizor in the Genitive case?

For masculine nouns that are objects (not people or animals), the Genitive ending can be either -a or -u. You often just have to memorize which one to use, but there are helpful patterns. For example, household appliances, electronics, and tools almost always take -a.

Why does the word for room change to pokoju here?

We are stating a location using the preposition w (in), which requires the Locative case. The basic dictionary form is pokój (room), but when we apply the Locative case rules, it becomes pokoju.

What happened to the letter ó from pokój when it changed to pokoju?

This is a very common Polish spelling rule! The letter ó in the final syllable of a word often changes to a regular o when a grammatical ending is added to the word. You will see this vowel shift happen all the time.