Reasoning with 〜のだ

One of the trickier Japanese grammar structures for English natives to get used to is のだ.

Consider the following observations:

The streets are wet.
The package that was left on my doorstep is not there.
My smartphone won’t turn on.

These are incomplete observations. We have observed the effect, but we do not know the cause.

Since the cause is unknown, we are left to reason what the cause is likely to have been.

It must have rained. (That’s why the streets are wet.)
My neighbor must have swiped another one of my packages. (That’s why the package is missing from my doorstep.)
The battery must have stopped working. (That’s why my smartphone won’t turn on.)

This type of reasoning (known in English as abductive reasoning) is where you work out the reason (cause) for your observation (result).

Note that these reasons may not necessarily be correct.

Perhaps a street sweeper washed the streets, maybe I had my package sent to the wrong address, and it could be that I dropped my smartphone and a wire disconnected.

In English, there isn’t anything built into the grammar to denote this kind of reasoning.

In Japanese, there is.

“It rainedのだ.”
“My neighbor swiped my packageのだ.”
“The battery stopped workingのだ.”

This reasoning is intended to explain the cause of what we’ve observed. Thus, you’ll see this grammar referred to as “the explanatory の”.

Structurally, what’s happening is that the sentence is being turned into a noun phrase with の, then だ is added to turn it into a noun sentence: “It is (that) [phrase].” A close approximation in English is to say “It’s because [phrase]”:

“The streets are wet. It’s because it must have rained.”
“The package that was left on my doorstep is not there. It’s because my neighbor must have swiped it.”
“My smartphone won’t turn on. It’s because the battery stopped working.”

It’s also used when you have the complete observation (you know the cause/reason), and you are informing someone who has an incomplete observation: