Learn Polish: Ktoś ukradł wczoraj mój telefon. - Someone stole my phone yesterday.

The word ktoś (someone) is just the question word kto (who) with the suffix -ś attached. This suffix turns a question word into an indefinite pronoun. For example, if you add -ś to co (what), you get coś (something).

Why is the verb ukradł in the masculine form? Does ktoś always take a masculine verb?

Yes! Even if you don't know who the thief was, or if you later find out it was a woman, the pronoun ktoś is always grammatically masculine singular. Because of this, it always takes the masculine past tense ending -ł, as seen in ukradł.

The phone is the object being stolen. Why didn't mój telefon change its ending for the accusative case?

It actually is in the accusative case! However, telefon is an inanimate (non-living) masculine noun. For these types of nouns, the accusative form is identical to the basic dictionary form (the nominative case). The word mój (my) follows this exact same rule here.

Can I change the position of wczoraj (yesterday) in this sentence?

Absolutely. Polish word order is flexible. You could put it at the very beginning (Wczoraj ktoś ukradł mój telefon) if you want to emphasize that it happened yesterday, or at the end. Putting it right after the verb is a very neutral, natural spot.