Learn Hardcore Japanese: seito ha sensei no setumei ga wakariyasui to omoimasu. - The students think the teacher's explanation is easy to understand.

生徒は marks 生徒 as the topic: “As for the students / speaking about the students…”
〜と思います normally means “I think that 〜” (the subject I is just omitted).

So the most natural interpretation is:

“(I) think that the teacher’s explanation is easy to understand for the students.”

If you really want to say “The students think …”, you would usually make 生徒 clearly the subject of 思う, for example:

生徒は先生の説明が分かりやすいと思っています。
“The students think that the teacher’s explanation is easy to understand.”

The original sentence is more about the speaker’s opinion, from the students’ point of view.

Why does 生徒 use は, and 説明 use が?

This is a very common pattern in Japanese:

N1 は N2 が A (adjective)

It often means:

“For N1, N2 is A.”

Here:

生徒は – topic / “as for the students / for the students”
先生の説明が – grammatical subject of the adjective
分かりやすい – “easy to understand”

So:

生徒は先生の説明が分かりやすい。
“For the students, the teacher’s explanation is easy to understand.”

Then 〜と思います just adds “I think …” to that.

So:

は: marks the overall topic / viewpoint (the group we are talking about).
が: marks the thing that is actually easy to understand (the explanation).

Could we say 生徒は先生の説明は分かりやすいと思います using は for 説明 instead of が? What changes?

You can say 先生の説明は分かりやすい, but in this longer sentence it becomes awkward:

生徒は先生の説明が分かりやすいと思います。 ✅ natural
生徒は先生の説明は分かりやすいと思います。 ⚠️ feels odd in most contexts

Why?

Double は (生徒は … 説明は …) easily sounds like contrast:
- “As for the students, as for the teacher’s explanation, (I) think it’s easy to understand (as opposed to something else).” This nuance is usually not what you want here.
Inside a clause before と思います, が is more neutral and common for marking the subject of an adjective:
- X が分かりやすいと思います is a very standard pattern.

So in this sentence, 先生の説明が is the natural choice.
Use 先生の説明は分かりやすい when that whole thing is the main sentence, not a clause before と思います:

先生の説明は分かりやすいです。
“The teacher’s explanation is easy to understand.”

What exactly is the role of と in 分かりやすいと思います?

と here is the quoting particle.

It marks what is being thought as a kind of “quote” in the mind and connects it to 思います:

[先生の説明が分かりやすい] と思います。
“I think that [the teacher’s explanation is easy to understand].”

So:

The part before と is the content of the thought.
と思います is “(I) think”.

You use the same と with many “mental” or “saying” verbs:

〜と言います – say that 〜
〜と思います – think that 〜
〜と思っています – be thinking that 〜 (ongoing state)
〜と感じます – feel that 〜

Why is there no です or だ after 分かりやすい before と?

Because:

分かりやすい is an い‑adjective, and い‑adjectives do not need だ or です to be a complete predicate.
- 分かりやすい。 = “It is easy to understand.”
Before と思います, you normally use the plain form:
- Plain verb: 行くと思います (“I think I’ll go.”)
- Plain い‑adjective: 高いと思います (“I think it’s expensive.”)
- Noun / な‑adjective: 便利だと思います (“I think it’s convenient.”)

So:

分かりやすいと思います is correct (い‑adjectives stay as they are).
You would not say 分かりやすいですと思います – that is unnatural.

What does 分かりやすい literally mean, and how is it formed?

分かりやすい (わかりやすい) is:

The verb stem 分かり (from 分かる, “to understand / to be clear”)
plus the suffix やすい, which means “easy to do”

So:

分かる – to understand / be understandable
分かりやすい – “easy to understand”, “easy to grasp / clear”

This pattern is very productive:

読みやすい本 – a book that is easy to read
使いやすいアプリ – an app that is easy to use
覚えやすい単語 – a word that is easy to remember

So 先生の説明が分かりやすい means “the teacher’s explanation is easy to understand.”

What is the difference between 分かります and 分かりやすい here? Could we say 先生の説明が分かります instead?

They are different in focus:

分かります – “(I / they) understand”
分かりやすい – “is easy to understand”

So:

先生の説明が分かります。
“(I / they) understand the teacher’s explanation.”
→ Focus: the listener’s ability or result.
先生の説明が分かりやすいです。
“The teacher’s explanation is easy to understand.”
→ Focus: the quality of the explanation (it is clear / well‑explained).

In the original sentence, we are evaluating how clear the explanation is, so 分かりやすい is more natural than 分かります.

What does 先生の説明 mean exactly, and what is の doing?

先生の説明 literally is:

先生 – teacher
の – possessive / attributive marker
説明 – explanation

So it means:

“the teacher’s explanation”
or “the explanation given by the teacher”

The particle の links two nouns, with the first one modifying the second:

日本の文化 – Japanese culture
会社の社長 – the company’s president
友だちの本 – my friend’s book

Here, 先生の説明 is one noun phrase meaning “the teacher’s explanation.”

Is 生徒 singular or plural here? How do we know if it is “student” or “students”?

In Japanese, bare nouns like 生徒 usually do not mark singular or plural. The number is decided by context.

So 生徒は先生の説明が分かりやすいと思います could be understood as:

“I think the student finds the teacher’s explanation easy to understand.”
or
“I think (the) students find the teacher’s explanation easy to understand.”

If you really want to emphasize plural, you can say:

生徒たちは先生の説明が分かりやすいと思います。
“I think the students (plural) find the teacher’s explanation easy to understand.”

Can we omit 生徒は and just say 先生の説明が分かりやすいと思います? What changes?

Yes, you can absolutely say:

先生の説明が分かりやすいと思います。

This simply means:

“I think the teacher’s explanation is easy to understand.”

What changes:

With 生徒は:
- 生徒は先生の説明が分かりやすいと思います。
  “I think that, for the students, the teacher’s explanation is easy to understand.”
  → Emphasis on students’ perspective.
Without 生徒は:
- 先生の説明が分かりやすいと思います。
  “I think the teacher’s explanation is easy to understand.”
  → More general statement; no explicit mention of whose perspective.

So adding 生徒は shifts the sentence to “as for the students / from the students’ point of view.”

Why is the form 思います used, and how would this change in casual speech?

思います is the polite (ます‑form) of the verb 思う.

思う – to think
思います – polite: “(I) think”

The original sentence is in polite style:

生徒は先生の説明が分かりやすいと思います。

In casual speech, you would usually change it to the plain form:

生徒は先生の説明が分かりやすいと思う。

The rest of the sentence is already in plain form (分かりやすい), so only 思います → 思う changes when switching to casual style.