Learn Hardcore Japanese: sensei ha kono hon wo osusumesimasu.

先生は = "As for the teacher," / "The teacher (speaking about them)…"
It sets up 先生 as background information; the comment about the teacher is that they recommend this book.

You could say 先生がこの本をおすすめします, but:

先生は… sounds like neutral explanation, setting the teacher as the topic.
先生が… puts more focus on “the teacher” as the specific one who recommends it, sometimes with a feeling of contrast (e.g., "It’s the teacher (not someone else) who recommends this book").

In a simple, neutral sentence like this, 先生は is very natural.

What does the particle を do in this sentence?

を marks the direct object of the verb – the thing that is being acted on.

この本をおすすめします = “recommend this book.”
The action (おすすめします) is being done to この本, so を is used.

In Japanese, most basic transitive verbs (ones that act on something) use を before the object.

Why isn’t there any word for “the” in この本?

Japanese doesn’t have articles like “a” or “the”.

本 just means “book.”
この本 literally = “this book.”

Whether you translate it as “this book”, “this book here”, “this (particular) book”, or “this is the book (I’m talking about)” depends on context in English. Japanese leaves that nuance to context rather than marking it with articles.

What exactly is おすすめします? Is it a single word, and what does it literally mean?

Grammatically, おすすめします is:

お – an honorific / polite prefix
すすめ – from the verb すすめる (勧める), “to recommend, to urge”
します – the polite form of する, “to do”

So structurally it’s like “(polite) recommendation + do”, i.e. “(I) do a recommendation”, which functions as the verb “to recommend”.

In practice, learners can just treat おすすめします as one polite verb meaning “(I) recommend (it).”

What’s the difference between おすすめします and すすめます / 勧めます?

Both are used for “recommend,” and in many everyday contexts they are interchangeable.

すすめます / 勧めます
- Direct polite form of すすめる (勧める), “to recommend, to urge, to encourage.”
- Feels like a straightforward verb.
おすすめします
- Literally “(I) do a recommendation,” but functions the same.
- Has a slightly more polite / salesy / service flavor (very common in shops, restaurants: こちらをおすすめします “We recommend this one”).

For a teacher recommending a book, both:

先生はこの本をおすすめします。
先生はこの本を勧めます。

are acceptable. おすすめします is common and sounds polite and natural.

Why is 先生 used by itself? Can I say 先生さん?

You cannot say 先生さん. 先生 itself is already an honorific-like title.

先生 literally means “teacher,” but is also used like “Mr./Ms./Dr./Professor” for certain professions (teachers, doctors, etc.).
It can be used:
- as a title: e.g., 鈴木先生 = “Mr./Ms./Dr. Suzuki”
- as “the teacher” / “my teacher” when it’s clear who you mean.

In this sentence, 先生は… simply means “The teacher (we’re talking about)…” or “My teacher…” depending on context. No さん is added.

Can I say 先生がこの本をおすすめします instead? What’s the nuance difference from 先生は?

Yes, it’s grammatically correct, but the nuance shifts.

先生はこの本をおすすめします。
- 先生 is the topic: “As for the teacher, (they) recommend this book.”
- Neutral, explanatory tone.
先生がこの本をおすすめします。
- 先生 is the subject in focus: “It’s the teacher who recommends this book.”
- Can sound like you’re emphasizing that the teacher (and not someone else) is the one recommending it, depending on context.

In a simple textbook-style example, 先生は is more typical.

Can I change the word order, like 先生はおすすめしますこの本を?

Japanese word order is more flexible than English, but not completely free. The basic pattern is:

[Topic/Subject] + [Object(s)] + [Verb]

The verb usually comes at the end.

Natural patterns here:

先生はこの本をおすすめします。 (most standard)
この本を先生はおすすめします。 (emphasizes “this book” more)

Your example 先生はおすすめしますこの本を is grammatically odd because you put the を-marked object after the verb. That’s not standard in Japanese. Keep この本を before おすすめします.

How would I say “The teacher recommended this book” (past tense)?

Change おすすめします (polite non-past) to おすすめしました (polite past):

先生はこの本をおすすめしました。
= “The teacher recommended this book.”

Similarly, with 勧める:

先生はこの本を勧めました。
= also “The teacher recommended this book.”

How do I say “The teacher recommends this book to me / to the students”?

Add the indirect object with に:

“The teacher recommends this book to me.”
- 先生は私にこの本をおすすめします。
“The teacher recommends this book to the students.”
- 先生は学生にこの本をおすすめします。

Pattern:

[Giver] は [Receiver] に [Thing] をおすすめします。

Why are there spaces between the words here? Japanese usually doesn’t use spaces, right?

Correct: in normal Japanese writing, there are usually no spaces between words:

Real-world style: 先生はこの本をおすすめします。

The version with spaces:

先生　は　この　本　を　おすすめします。

is a teaching aid. The spaces are there just to help learners see the word boundaries and particles. In actual Japanese texts (books, websites, manga, etc.), you won’t see those spaces.

Could I drop some parts of the sentence, like just say この本、おすすめします?

Yes. Japanese often drops information that is obvious from context.

If everyone already knows you’re talking about what the teacher recommends, you might say:
- この本、おすすめします。
  = “(I / we / they) recommend this book.”
You could also drop この本 if it’s clear what “it” is:
- おすすめします。
  = “(I) recommend it.”

In a standalone textbook sentence, everything is spelled out (先生はこの本をおすすめします) so learners can clearly see each role (topic, object, verb). In natural conversation, people would shorten it depending on context.