Learn Hardcore Japanese: sensei no setumei ha, nihongo no bunpou no ii rei da to omoimasu. - I think the teacher's explanation is a good example of Japanese grammar.

先生の説明
- Literally: teacher + の + explanation
- Meaning: the teacher’s explanation (the explanation given by the teacher)
日本語の文法のいい例
Break it down:
- 日本語の文法 = the grammar of Japanese (language)
- 日本語の文法のいい例 = a good example of Japanese grammar

So each の shows that what comes before it modifies or belongs to what comes after it:

先生 → 説明 (the explanation belonging to / made by the teacher)
日本語 → 文法 (the grammar belonging to / of Japanese)
文法 → いい例 (a good example relating to that grammar)

Why is は used after 先生の説明 instead of が?

は marks the topic of the sentence – what you are talking about.
が typically marks the subject and often introduces new or emphasized information.

先生の説明は、日本語の文法のいい例だと思います。
“As for the teacher’s explanation, (I) think (it) is a good example of Japanese grammar.”

Using は suggests:

We already know what explanation we’re talking about (it’s established in the conversation).
The sentence is giving information about that explanation.

If you said:

先生の説明が、日本語の文法のいい例だと思います。

It would sound more like:

“I think it is the teacher’s explanation that is a good example of Japanese grammar,”
putting more contrast/emphasis on 先生の説明 as opposed to someone else’s explanation.

In most neutral contexts here, は is more natural.

Why is だ used before と思います? Can I say ですと思います instead?

In this pattern, you normally use the plain form before と思います.

For a noun or な-adjective predicate:
- Noun + だと思います
- な-adjective + だと思います

Here, いい例 is being treated as a noun phrase (an example that is good), so you get:

いい例だと思います = “I think (it) is a good example.”

ですと思います is generally unnatural in this structure.
The usual pattern is:

X だと思います。
(I) think (it) is X.

Politeness is carried by 思います, which is the polite form of 思う. The part inside …だと is in plain form, even in polite sentences. So:

Correct / natural: いい例だと思います。
Unnatural: いい例ですと思います。

What is the role of と in だと思います?

と here is the quotative particle. It marks what is being “quoted” or thought/said.

Structure:

[Clause] + と + 思います
= “I think that [Clause].”

In this sentence:

先生の説明は、日本語の文法のいい例だ
is the clause “(The) teacher’s explanation is a good example of Japanese grammar.”
…だ
- と
  - 思います
    = “I think that (it) is a good example of Japanese grammar.”

So と connects the statement “日本語の文法のいい例だ” with the verb 思います (“think”), just like “that” in “I think that it is a good example.”

Who is doing the “thinking” in 思います? There is no 私 in the sentence.

Japanese often omits pronouns like 私 (“I”) if they are clear from context.

In this sentence:

思います is in the first person unless there is a clear reason to interpret it otherwise.
In normal conversation, a speaker saying …と思います is understood as “I think …”.

So even though 私 is not said, the sentence is naturally understood as:

（私は）先生の説明は、日本語の文法のいい例だと思います。
“(I) think the teacher’s explanation is a good example of Japanese grammar.”

How is the long phrase 日本語の文法のいい例 structured? What is the order compared to English?

Japanese builds noun phrases from left to right, with modifiers coming before the main noun.

Breakdown:

日本語 = Japanese (language)
日本語の文法 = the grammar of Japanese
日本語の文法のいい例:
- 日本語の文法 = Japanese grammar
- 日本語の文法のいい = good (in relation to Japanese grammar)
- 日本語の文法のいい例 = a good example of Japanese grammar

The head noun is the last one: 例 (example).

So the order is: > [Japanese] → [grammar] → [good] → [example]

In English we reverse some parts: > a good example of Japanese grammar

So you can think of it as: all the information that would come after “example” in English (…of X, for Y, etc.) comes before 例 in Japanese.

Is 先生の説明 “the teacher’s explanation” or “an explanation for the teacher”? How do I know?

X の Y is flexible; it can indicate possession, origin, content, etc. Here, context and common sense guide the interpretation:

先生の説明 is most naturally:
- “the teacher’s explanation”
  → explanation given by the teacher.

If it were “an explanation for the teacher,” Japanese would more likely say something like:

先生への説明 (“explanation to/for the teacher”)
先生に対する説明 (similar nuance)

Because の easily expresses “belonging to” or “done by,” and “teachers give explanations” is natural, 先生の説明 is interpreted as the explanation made by the teacher.

Why is いい used here? Could I use よい instead, and is there any difference?

いい and よい are essentially the same adjective in modern Japanese.

いい例 and よい例 both mean “a good example.”

Nuance:

いい is more common in everyday spoken Japanese and neutral writing.
よい can sound a bit more formal, literary, or stiff, and appears more in writing (instructions, official documents, etc.) or set phrases.

In this sentence, either is grammatically fine:

日本語の文法のいい例だと思います。
日本語の文法のよい例だと思います。

The first feels more casual/neutral; the second, slightly more formal.

Why isn’t there an を after 先生の説明? In English “I think the teacher’s explanation is a good example,” “the teacher’s explanation” feels like an object.

In Japanese, 思います in this pattern doesn’t take a direct object with を. Instead, it takes a clause + と as its “object.”

Structure here is:

[先生の説明は、日本語の文法のいい例だ] + と + 思います。

So:

先生の説明は → topic within the inner clause
…いい例だ → predicate of that clause
…だと → marks that whole clause as the content of the thought
思います → “(I) think”

If you used を, you’d change the structure to something like:

先生の説明を、日本語の文法のいい例だと思います。

This is grammatically possible and would mean something like:

“I think the teacher’s explanation is a good example of Japanese grammar,” with more emphasis on the teacher’s explanation as the direct “thing” you’re judging.

But in the original sentence, the structure is “(As for) the teacher’s explanation, (I) think (it) is a good example…”, so は is used, and the clause as a whole is the object of 思います, not 先生の説明 alone.

Can I drop だ and say 日本語の文法のいい例と思います?

For sentences where a noun (or な-adjective) is the predicate, the standard pattern before と思います is to include だ:

いい例だと思います。 ✓ natural
いい例と思います。 ✕ generally considered incorrect/unnatural in standard Japanese

You may sometimes hear だ omitted in fast, casual speech, especially more with な-adjectives than with pure nouns, but for learners and in correct written Japanese, you should keep だ here.

So:

Use 日本語の文法のいい例だと思います。
Avoid 日本語の文法のいい例と思います。 (textbook-incorrect)

Why is there a comma after 先生の説明は、? Is it required?

The comma after 先生の説明は、 is mainly for readability and rhythm, not strict grammar.

Japanese commas (、) often mark pauses or help group information.
After a topic marked by は, it’s very common to place a comma, especially in writing:
- X は、Y です。

You could also write:

先生の説明は日本語の文法のいい例だと思います。

This is still correct. The meaning doesn’t change; the comma just makes it slightly easier to parse, especially in longer sentences.