Learn Turkish: Polis, göçmene kaçak eşyayı beyan etmesini söyledi. - The police told the immigrant to declare the illegal goods.

When expressing a reported command like "told someone to do something", Turkish uses a verbal noun instead of the infinitive. You take the root beyan et-, add the verbal noun suffix -me, and then the possessive -si to show who is supposed to do it (his/her declaring). Finally, you add the accusative case -ni because this entire action clause is the direct object of the main verb söyledi.

Why does göçmen (immigrant) have the dative suffix -e (göçmene)?

The verb söylemek (to tell/say) requires the dative case for the person you are speaking to. While English says "told the immigrant", the Turkish structure translates more literally to "said to the immigrant".

Both eşyayı and beyan etmesini have the accusative case. Why are there two accusative objects in one sentence?

Each verb or verbal noun in the sentence takes its own object. Eşyayı (the goods) gets the accusative suffix -yı because it is the specific direct object of the action beyan etmek (to declare). Meanwhile, beyan etmesini gets the accusative suffix -ni because that entire action is the direct object of the main verb söyledi (told).

The English translation says "goods" (plural), but eşyayı doesn't have the plural suffix -lar. Why?