Learn Croatian: Zašto ptice ne piju mlijeko? - Why do the birds not drink milk?

The negative word ne goes directly in front of the verb it negates. Here, it sits right before piju to make ne piju (do not drink). You cannot separate ne and the verb with another word.

The verb for "to drink" is piti. Why does it change to piju here?

The verb piti has a slight irregularity in the present tense: it adds a j to the stem. Because the subject is "the birds", which is a "they" (third-person plural) perspective, we use the ending -u, making it piju.

Since mlijeko (milk) is the object of the sentence, shouldn't its ending change?

It doesn't change because mlijeko is a neuter noun (it ends in -o). In Croatian, neuter nouns look exactly the same whether they are the subject doing the action or the object receiving the action.

How do we know ptice is plural?

The singular word for "bird" is ptica, which is a feminine noun because it ends in -a. To make a feminine noun plural in Croatian, you change the -a to an -e, which gives us ptice.