Learn Polish: Wczoraj mój syn dostał świetny prezent.

Dostał is the past tense form of the verb dostać (to get). In Polish, past tense verbs change based on the gender of the person or thing doing the action. Because syn (son) is masculine, the verb takes the masculine past tense ending -ł.

Świetny prezent (a great gift) is the object of the verb. Why didn't its endings change?

You are right that it is the direct object, which means it takes the accusative case. However, for masculine inanimate nouns (objects and things that are not alive, like a gift), the accusative form looks exactly the same as the basic dictionary form. Therefore, świetny prezent does not change.

Why is the word for 'my' mój and not moja or moje?

Possessive words like 'my' have to match the gender of the noun they belong to. Because syn is a masculine noun, we use the masculine form mój. If the sentence was about a daughter (córka), you would use the feminine form moja.

Can I put wczoraj (yesterday) somewhere else in the sentence?

Yes, Polish word order is quite flexible. It is very common to put time words like wczoraj at the beginning of the sentence to set the scene, but saying Mój syn dostał wczoraj świetny prezent is also perfectly natural and correct.