Learn Russian: В доме порядок.

В доме is in the prepositional singular because this sentence is about just one house. To use the prepositional plural you are currently learning, you would say В домах порядок (There is order in the houses), adding the -ах ending to the noun.

Where is the verb "to be" (is) in this sentence?

In Russian, the present tense verb for "to be" is almost always skipped. The sentence literally translates to "In the house [is] order." If you were talking about the past, you would need to add the past tense verb: В доме был порядок (The house was in order).

Why does the sentence start with the location (В доме) instead of the subject (порядок)?

Russian sentence structure often places the location first to set the scene, leaving the "new" or most important information for the end of the sentence. Starting with В доме emphasizes that what exists there is order. If you said Порядок в доме, it would sound more like "The order is in the house."

How does the word порядок relate to the common phrase Всё в порядке?

Порядок is a noun meaning "order" or "tidiness." In our sentence, it is the subject, so it stays in the nominative case. When you say Всё в порядке ("Everything is fine" or "Everything is in order"), you are using the same word, but it takes the prepositional case ending -е because of the preposition в.