Learn Turkish: Eşit çukurlara birer kazık gömüyoruz. - We are burying one stake each into the equal holes.

It is the distributive suffix, meaning each. By adding -er to bir (one), we get birer, which means one each. In this sentence, it tells us that exactly one stake is being placed into every single hole.

If the distributive suffix can be -şar or -şer, why is it just -er here?

The buffer consonant ş is only added when the number ends in a vowel, such as in ikişer (two each) or yedişer (seven each). Because the word bir ends in a consonant, it directly takes the -er suffix.

Why do we use the dative case in çukurlara instead of the locative case like çukurlarda?

In English, you usually bury something in a hole, which sounds like a fixed location. However, the Turkish verb gömmek (to bury) focuses on the physical movement of an object into a place. Therefore, it requires the dative suffix -a (towards/into), giving us çukurlara.

Why does kazık (stake) remain in its bare form without an accusative suffix?

Because it acts as an indefinite object. The sentence describes burying general stakes (one in each hole), rather than specific, previously identified stakes. When a direct object is non-specific, it does not take the accusative suffix.