Learn Turkish: İşçiler her sıraya ikişer fidan taşıyor. - The workers are carrying two saplings to each row.

The number for two is iki. Since it ends in a vowel, we need the buffer letter -ş- before adding the distributive suffix -er (which follows 2-way vowel harmony). So iki becomes ikişer, meaning two each or two per row.

Why isn't the word for sapling plural (fidanlar) since they are carrying two of them?

In Turkish, whenever a noun is modified by a number or a quantity word (like the distributive ikişer), the noun always stays in its singular form. You say ikişer fidan (two saplings each), never ikişer fidanlar.

Why is the verb taşıyor in the singular form if the subject (işçiler) is plural?

When the subject of a sentence is a plural person (like işçiler, workers), the verb can take either the singular or plural form. Both taşıyor and taşıyorlar are perfectly correct here.

Why is there a -ya at the end of sıraya?

The verb taşımak (to carry) shows movement toward a place, so the destination needs the dative case (meaning to or towards). The word for row is sıra. Because it ends in a vowel, we add the buffer -y- before the dative suffix -a, making sıraya (to the row).