Learn Turkish: Bütün eşyaları zorlana zorlana kamyona sığdırdık. - We fit all the items into the truck with great difficulty.

This uses the repeated manner converb. The root is zorlanmak (to struggle or have difficulty). We drop the -mak and add -a to the root, repeating it to form zorlana zorlana. It describes how the main action was done: we made the items fit 'while struggling repeatedly' or 'with difficulty'.

What is the difference between sığmak and the verb used here, sığdırmak?

sığmak means to fit on its own, for example if you said 'The items fit in the truck.' But here, we are making the items fit. Adding the causative suffix -dır creates sığdırmak, meaning to make something fit or to squeeze something in.

Why does eşyaları have that final -ı on the end?

The word is eşya (item or thing), pluralized to eşyalar. Because these are specific items receiving the action of the verb (we are fitting these specific items), the word needs the accusative case ending -ı, making it eşyaları.

English says we fit the items 'into' the truck. Why does Turkish use the dative ending -a on kamyona?