Bài tập này khá dễ, nhưng viết lại bằng tiếng Việt thì khó hơn tôi nghĩ.

In Vietnamese, demonstratives like này (this) usually come after the noun.

• bài tập này = this exercise
• cuốn sách này = this book

This is the normal Vietnamese word order, even though English does the opposite.

Bài tập is a very common compound noun meaning exercise, task, or assignment, depending on context.

• bài can mean a piece of work, lesson, or set item
• tập relates to practice

Together, bài tập means a practice task or exercise. In this sentence, this exercise is the most natural translation.

Khá means quite, fairly, or rather.

So:

• khá dễ = quite easy / fairly easy

It is usually weaker than rất (very).
So khá dễ sounds more moderate than rất dễ.

Nhưng means but. It connects two contrasting ideas:

• Bài tập này khá dễ = the exercise is quite easy
• nhưng... khó hơn tôi nghĩ = but ... it is harder than I thought

So the sentence contrasts the exercise itself seeming easy with the act of rewriting it in Vietnamese being more difficult.

Viết lại literally means write again.

In real usage, it can mean:

• rewrite
• write out again
• rephrase in writing

In this sentence, it suggests rewriting it in Vietnamese, not just writing something unrelated. Depending on context, it may feel close to putting it into Vietnamese in written form.

Vietnamese often leaves out subjects or objects when they are already clear from context.

Here, after Bài tập này khá dễ, the listener already knows what is being discussed. So viết lại bằng tiếng Việt is understood as something like:

• rewriting it in Vietnamese
• to rewrite it in Vietnamese

English usually needs to make this more explicit, but Vietnamese often does not.

It could, but Vietnamese commonly omits the object if it is obvious.

The implied object is the exercise, or the content of the exercise. So:

• viết lại bằng tiếng Việt = rewrite it in Vietnamese

A fuller version might be:

• nhưng viết lại nó bằng tiếng Việt thì khó hơn tôi nghĩ

But that version can sound a bit heavier. Omitting nó is very natural here.

With languages, Vietnamese often uses bằng + tiếng + language to mean in / using that language.

So:

• bằng tiếng Việt = in Vietnamese
• bằng tiếng Anh = in English

Here, bằng shows the language used as the medium of expression.

Here, thì does not indicate tense. It helps mark the topic and sets up the comment about it.

• viết lại bằng tiếng Việt thì khó hơn tôi nghĩ

This has a feeling like:

• as for rewriting it in Vietnamese, it is harder than I thought

So thì helps frame rewriting it in Vietnamese as the thing being commented on. The sentence could exist without thì, but with thì it sounds more natural and structured.

Hơn is the comparative word meaning more than.

• khó = difficult
• khó hơn = more difficult / harder

So:

• khó hơn tôi nghĩ = harder than I thought

Literally, it looks like harder than I think, but Vietnamese often does not mark tense the way English does. In context, English naturally uses thought.

Vietnamese verbs do not have built-in tense endings like English verbs do. Time is often understood from context.

So tôi nghĩ can mean:

• I think
• I thought

depending on the situation.

In this sentence, English prefers I thought because the speaker is comparing reality now with an earlier expectation.
If you want to make the past idea more explicit in Vietnamese, tôi đã nghĩ is possible, but tôi nghĩ is still very natural.

Yes, that is the basic structure.

The contrast is:

1. The exercise itself seems fairly easy
2. But rewriting it in Vietnamese is harder than expected

So the speaker is not contradicting themselves. They are saying that understanding or doing the exercise may be easy, while expressing it again in Vietnamese is more difficult.

Yes. That is a very natural translation.

Even though the Vietnamese says tôi nghĩ literally, English often chooses I expected or I thought because that sounds more natural in this context.

Good natural translations include:

• This exercise is quite easy, but rewriting it in Vietnamese is harder than I thought.
• This exercise is fairly easy, but putting it into Vietnamese is harder than I expected.

Both capture the meaning well.