Learn Turkish: Komutan birlik için kural koyar. - The commander sets a rule for the unit.

In Turkish, when a direct object is indefinite (meaning a rule or rules in general, rather than the specific rule), it stays in its bare dictionary form. Since the commander is setting a general rule, kural doesn't take the accusative case.

Why is the verb koymak (to put) in the aorist tense here (koyar)?

The aorist tense (using the -ar ending here) is used for general truths, habits, and typical characteristics. The sentence states a general fact about a commander's role: a commander sets rules.

In English we make or set a rule. Does koymak literally mean that?

The primary meaning of koymak is to put or to place. In Turkish, rules aren't made using yapmak; they are put or laid down. Kural koymak is the standard natural phrase for setting a rule.

Why is it just birlik için? Doesn't için (for) require a special ending on the word before it?

The postposition için pairs with the bare dictionary form of regular nouns. It only requires the genitive case when used with pronouns, like benim için (for me) or kimin için (for whom). Since birlik is a regular noun, it stays exactly as it is.