Learn Russian: Тяжело распределять продукты по четырём складам. - It is hard to distribute products across four warehouses.

That rule only applies when the number is in the Nominative or inanimate Accusative case! When a number like 2, 3, or 4 is in an oblique case (like the Dative case here), the number declines, and the noun goes into the matching plural case. So we use the Dative of четыре (четырём) and the Dative plural of склад (складам).

Why do we use the preposition по here, and why does it trigger the Dative case?

The preposition по paired with the Dative case is often used to show distribution or movement across multiple locations. Here, по четырём складам expresses distributing something "across" or "among" four warehouses.

Why use the imperfective verb распределять here instead of the perfective?

Using the imperfective распределять after an adverb like тяжело (it is hard) usually points to a general, repeated process. The speaker is saying that the ongoing task of distributing products is difficult in general. If they were talking about one specific shipment that needs to be distributed right now, they would likely use the perfective распределить.

There is no word for "It" in the Russian sentence. How do we know it means "It is hard"?

Russian frequently uses impersonal sentences, which are sentences without a formal subject like "it." The adverb тяжело acts as the main predicate all by itself, meaning "it is hard" or "it is difficult," before introducing the infinitive verb.