Learn Hardcore Japanese: yosyuu to hukusyuu no baransu ga taisetu da to omoimasu. - I think the balance between preparation and review is important.

予習（よしゅう）: preparation before a class or lesson
- e.g. reading the textbook in advance, checking new vocabulary, previewing grammar
復習（ふくしゅう）: review after a class or lesson
- e.g. going over your notes, doing practice questions, re-reading what was covered

They’re often used as a set phrase: 予習と復習 = “preparation and review.”

Why is と used between 予習 and 復習? Could it be や instead?

Here, と simply means “and” linking two nouns:

予習と復習 = “preparation and review”

You could say 予習や復習, but it slightly changes the nuance:

A と B: A and B specifically (a complete pair/list)
A や B: A and B among other similar things (an incomplete list, “A, B, and so on”)

Since 予習と復習 are being contrasted as a clear pair whose balance matters, と is more natural.

What does の do in 予習と復習のバランス?

の here marks a possessive or relationship:

予習と復習のバランス
= “the balance of preparation and review”
= “the balance between preparation and review”

So structurally it’s:

[予習と復習] の [バランス]
“the balance of [preparation and review].”

Why is が used with バランス instead of は?

が marks the grammatical subject and tends to highlight or “present” the thing as important in this sentence:

予習と復習のバランスが大切だ
“The balance between preparation and review is (what is) important.”

You could say バランスは大切だ, but then バランス is more like the topic:

バランスは大切だ
“As for balance, it’s important.” (more general-sounding)

With the full sentence:

予習と復習のバランスが大切だと思います。
sounds like you’re specifically asserting that this balance is what’s important.
予習と復習のバランスは大切だと思います。
sounds a bit more like a general comment about that balance.

Both are grammatical; が feels a bit more neutral and “stated as a fact being observed.”

What is the nuance of 大切 here? How is it different from 重要 or 大事?

All three can mean “important,” but they differ in nuance:

大切（たいせつ）
- “important, precious, something to be cherished/taken care of”
- often has a slightly emotional or caring feel
- common in both speech and writing
大事（だいじ）
- very close to 大切, often interchangeable in casual speech
- can also mean “a big matter / serious issue” depending on context
重要（じゅうよう）
- more formal, logical, “significant, crucial”
- common in essays, reports, exams: 重要なポイント, 重要な問題

In this sentence, 大切 suggests “this balance is something you should really value / take care of,” which fits a teacher or learner’s advice tone well.

Why is there だ before と in 大切だと思います? Can you say 大切と思います?

だ is needed because 大切 is a na-adjective. When you use a na-adjective in front of と (思う) to make a clause like “I think (that) …,” you typically use its plain form:

大切だ
- と
  - 思います
    “(I) think (it) is important.”

Dropping だ (大切と思います) is generally unnatural in standard Japanese.
(There are a few special cases with other na-adjectives, but not here.)

So:

✅ 大切だと思います。
❌ 大切と思います。

Why is 思います in the polite form? How would the casual version look?

思います is the polite -ます form of 思う:

思う → plain form (casual)
思います → polite form (used in most formal / semi-formal contexts)

Casual versions of the whole sentence would be:

予習と復習のバランスが大切だと思う。
Or even with spoken-topic って:
予習と復習のバランスって大切だと思う。

The meaning is the same; only politeness and tone change.

Who is the subject of 思います? Why isn’t 私 written?

In Japanese, the subject is often omitted when it’s clear from context. In this kind of sentence:

（私は）予習と復習のバランスが大切だと思います。

the subject is understood to be 私 (“I”). You only explicitly say 私は if you need to emphasize who thinks this, or if it would be unclear.

So this sentence is naturally understood as:

“I think the balance between preparation and review is important.”

Could the sentence be rearranged as バランスが大切だと思います without 予習と復習の at the beginning?

If you say just:

バランスが大切だと思います。

it means:

“I think balance is important.”

This is grammatical but less specific.
The original:

予習と復習のバランスが大切だと思います。

tells you what kind of balance is important: specifically, the balance between preparation and review. So 予習と復習の is important information, not just movable fluff.

(You could move the whole phrase elsewhere in a longer sentence, but within this simple sentence, its position is already natural.)

Why is バランス written in katakana instead of kanji? Is there a Japanese word for it?

バランス is a loanword from English “balance,” so it’s written in katakana, which is used for:

foreign loanwords
many technical terms
certain onomatopoeia and stylized words

There are native/Japanese-origin alternatives in some contexts, like:

均衡（きんこう） – equilibrium (more formal/technical)
調和（ちょうわ） – harmony (more about things fitting together nicely)

But in everyday conversation about study habits, バランス is the most natural and common word to use.

How do you read and pronounce the whole sentence?

In kana:

よしゅうとふくしゅうのバランスがたいせつだとおもいます。

In one smooth phrase (spacing just for clarity):

よしゅうとふくしゅうのバランスがたいせつだとおもいます。

In romaji:

Yoshū to fukushū no baransu ga taisetsu da to omoimasu.

Stress is relatively even across syllables; Japanese doesn’t have English-style word stress.

Is there any nuance added by using と思います instead of just です? For example, why not 大切です?

Yes, there is a nuance difference:

大切です。
→ “It is important.” (more direct, assertive statement)
大切だと思います。
→ “I think it is important.” (softer, more subjective/opinion-like)

Using と思います often softens what you say, making it sound:

more like your personal view
less like you are stating an absolute fact

This is very common in polite Japanese, especially in teaching, meetings, and conversations where you don’t want to sound too blunt or bossy.