Learn Hardcore German: Wir sparen Wasser und senken so die Kosten.

Because Wasser is an uncountable (mass) noun in German. When talking about water in general—not a specific quantity or container—you omit the article.

What case is die Kosten in, and why is it die?

die Kosten is accusative plural. It’s the direct object of the verb senken. In German, the article for plural accusative is die, which coincides with the nominative plural form.

What role does so play in this sentence?

so is an adverb meaning “thus,” “thereby,” or “in doing so.” It links the action of saving water to the resulting action of lowering costs.

Why is so placed before die Kosten rather than at the end?

German places adverbs like so in the “middle field” immediately after the verb (when it’s modifying that verb). The order here is: verb → adverb → object.

Why isn’t there a comma before und?

Here und connects two verbs (a compound predicate) within the same clause. German only requires a comma before und when it joins two independent clauses each with its own subject.

Why doesn’t the second verb senken go to the end of the clause?

Because und links two verbs under the same subject wir, creating one clause with a compound predicate. The V2 (verb-second) rule applies only once at the start; both verbs appear in sequence after the subject.

Could I use reduzieren instead of senken?

Yes. Kosten reduzieren and Kosten senken are both correct. senken literally means “to lower,” while reduzieren means “to reduce.” They are largely interchangeable with Kosten, though senken is a bit more idiomatic for “lowering” prices or costs.

What’s the difference between using so here and using damit or um…zu?

so indicates a direct result (“we save water and thereby lower costs”).
damit or um…zu express purpose (“in order to”).
Example with purpose: Wir sparen Wasser, damit wir die Kosten senken.
Example with result: Wir sparen Wasser und senken so die Kosten.