Learn Hardcore Japanese: sono keisatukan ha, kono mati no tizu ni totemo kuwasii desyou. - That police officer is probably very familiar with this town’s map.

この = this (near the speaker)
- Example: この町 = this town (where I am / we are now)
その = that (near the listener, or already known in the conversation)
- その警察官 = that police officer (you know, the one we mentioned / the one near you)
あの = that over there (far from both speaker and listener, or more distant / not directly involved)

In this sentence, その警察官 suggests:

We both know which police officer we’re talking about, or
The officer is “on your side” / near you, or
The officer was mentioned earlier in the conversation.

Why is there a は after 警察官? What does it do?

は is the topic marker.

その警察官は = “As for that police officer,” / “That police officer…”

It tells you what the topic of the sentence is—what we’re talking about—before giving information about it.

So the structure is:

その警察官は (topic: that police officer)
この町の地図にとても詳しいでしょう (comment: is probably very familiar with the map of this town)

What does この町の地図 literally mean, and what is the role of の?

この町の地図 literally breaks down as:

この町 = this town
の = “of” / possessive or descriptive linker
地図 = map

So この町の地図 = “the map of this town.”

の links the two nouns, similar to English “X of Y” or “Y’s X”:

東京の地図 = Tokyo’s map / a map of Tokyo
日本の歴史 (にほんのれきし) = Japan’s history

Here, の connects 町 (town) and 地図 (map) in exactly that way.

Why is the particle に used after 地図? Why not を?

The verb-like word here is the adjective 詳しい (detailed / knowledgeable), and it normally takes に, not を.

Pattern:

X に詳しい = “to be well-informed / very familiar / knowledgeable about X”

So:

この町の地図に詳しい = “(someone) is very familiar with the map of this town.”

Some other examples:

日本の歴史に詳しい = to know a lot about Japanese history
コンピューターに詳しい = to be good with computers / know a lot about computers

Using を here would be ungrammatical because 詳しい doesn’t take を; it specifically uses に to mark the field/subject of knowledge.

What exactly does 詳しい mean here? Is it just “knows well”?

詳しい (くわしい) means:

detailed; in detail; well-informed; very familiar with; knowledgeable about

In this sentence:

この町の地図にとても詳しい
≈ “is very familiar with the map of this town”
≈ “knows the map of this town in detail”

It’s stronger than just 知っている (“to know”); it implies deep or detailed knowledge.

Compare:

この町の地図を知っています。= I know this town’s map. (exists in my knowledge)
この町の地図に詳しいです。= I know this town’s map in detail / I’m very familiar with it.

Why does the sentence end with でしょう? Does it mean future “will”?

In this sentence, でしょう is not future tense. It has the nuance of:

“probably / I suppose / I guess / I expect”, or
Softening the statement, making it less direct.

So:

その警察官は、この町の地図にとても詳しいでしょう。
= “That police officer is probably very familiar with the map of this town.”
= “I’d say that police officer knows the map of this town very well.”

If you used です instead:

…詳しいです。 = more direct: “That officer is very familiar with…”
…詳しいでしょう。 = softer, more like a conjecture or polite guess.

Context decides if it’s:

A guess: “He must be really familiar with it.”
Or a softening: “He’s very familiar with it, I think / you know.”

Is this sentence polite or casual? How formal does でしょう sound?

The sentence is in polite form, because of でしょう.

Politeness levels:

Casual: その警察官は、この町の地図にとても詳しいよ。
Polite: その警察官は、この町の地図にとても詳しいです。
Polite with conjecture/softening: その警察官は、この町の地図にとても詳しいでしょう。

でしょう is polite, but very common in spoken Japanese. It doesn’t sound stiff; it sounds natural and a bit softer than a blunt statement.

Can I say とても詳しいです instead of とても詳しいでしょう? What’s the difference?

Yes, you can. Both are grammatically correct but the nuance changes:

とても詳しいです。
- Straight, factual statement
- “He/She is very familiar (I state this as fact).”
とても詳しいでしょう。
- Suggests probability, inference, or softening
- “He/She is probably very familiar,”
- or “He/She is very familiar, I’d say / wouldn’t you agree?”

Use です if you want to sound certain.
Use でしょう if:

You’re inferring from what you know,
Or you want to sound less blunt.

What does とても add? Could I use something else like すごく or とてもよく?

とても is an adverb meaning “very”.

とても詳しい = very detailed / very familiar

You could replace it with:

すごく詳しい (more casual, conversational: “really familiar”)
非常に詳しい (ひじょうに; more formal: “extremely familiar”)
かなり詳しい (fairly / quite familiar)

You can also say:

とてもよく知っている = “knows very well”
but that changes the structure to use 知る instead of 詳しい.

Is the word order fixed? Could I move その警察官は or この町の地図に?

Japanese word order is fairly flexible as long as particles stay with their words.

Your original:

その警察官は、この町の地図にとても詳しいでしょう。

Some natural variations:

この町の地図に、その警察官はとても詳しいでしょう。
その警察官は、とてもこの町の地図に詳しいでしょう。 (a bit marked, but possible)

What must remain:

地図に詳しい (the に must stay with 地図, because 詳しいに is the pattern)

Changing the order usually changes emphasis, not basic meaning. The original order is the most neutral and natural.

Why don’t we add さん after 警察官? Is that rude?

You can say:

その警察官さんは → sounds odd; さん is usually not attached to job titles like this.
その警察官の方 (かた) → more polite (“that police officer (person)”).

For occupations, it’s common not to add さん directly:

先生 (teacher) can be used alone: 先生は…
警察官 also: その警察官は…

It’s not inherently rude in narration or when talking about “a police officer” in general. To be extra polite about a specific officer you might say:

あの警察官の方は、この町の地図にとても詳しいでしょう。

How do you read 警察官 and 詳しい, and are there any common mistakes with them?

Readings:

警察官 = けいさつかん (keisatsukan)
- 警 (けい), 察 (さつ), 官 (かん)
詳しい = くわしい (kuwashii)

Common points:

Don’t confuse 詳しい (くわしい) with 凄い (すごい) or 上手 (じょうず).
- 詳しい = detailed / knowledgeable about a field or subject
- 上手 = skillful at doing something
- 凄い = amazing / great

Examples:

彼は地図に詳しい。= He knows maps well.
彼は運転が上手だ。= He is good at driving.

What is the basic literal structure/translation of the whole sentence?

Breaking it down literally:

その警察官は = As for that police officer,
この町の地図に = in regard to the map of this town / about this town’s map,
とても詳しい = is very detailed / very knowledgeable,
でしょう = probably / I suppose / I’d say.

Putting it together:

“As for that police officer, (he/she) is probably very knowledgeable about the map of this town.”

More natural English:

“That police officer is probably very familiar with the map of this town.”