Tôi nghĩ đi xe buýt đến quán đó dễ hơn đi bộ khi trời mưa.

Tôi nghĩ means I think.

It shows that this is the speaker’s opinion, not a hard fact. In Vietnamese, it works much like English:

• Tôi nghĩ... = I think...
• Tôi thấy... = I feel / I find...
• Theo tôi... = In my opinion...

So the speaker is saying that, in their view, taking the bus to that place is easier than walking when it rains.

Usually, no. Vietnamese often goes straight from I think to the idea itself:

• Tôi nghĩ đi xe buýt... dễ hơn...

This is very normal. English can say I think that..., but Vietnamese usually does not need a separate word there.

If you translate word-for-word too closely from English, you may expect an extra connector, but Vietnamese commonly leaves it out.

Because đi is a very common verb meaning to go or to travel, and it also appears in set travel expressions.

In this sentence:

• đi xe buýt = go by bus / take the bus
• đi bộ = go on foot / walk

So even though English uses different verbs like take and walk, Vietnamese often keeps đi and then adds the method of movement.

That is why you see đi more than once.

Yes, very roughly it is go by bus.

The pattern is:

• đi + means of transport

For example:

• đi xe buýt = go by bus / take the bus
• đi xe đạp = go by bicycle
• đi xe máy = go by motorbike
• đi tàu = go by train
• đi máy bay = go by plane

So đi xe buýt đến quán đó means go to that place by bus.

A native English speaker may expect a verb like take, but Vietnamese does not need that here.

The pattern is:

• A + adjective + hơn + B

Here:

• A = đi xe buýt đến quán đó
• adjective = dễ
• hơn = more / than
• B = đi bộ

So:

• đi xe buýt đến quán đó dễ hơn đi bộ
• literally: going by bus to that place is easier than going on foot

In Vietnamese, hơn is the normal word used to make comparisons:

• nhanh hơn = faster
• rẻ hơn = cheaper
• khó hơn = more difficult

The whole action phrase đi xe buýt đến quán đó acts as the subject-like element.

So the structure is basically:

• [đi xe buýt đến quán đó] dễ hơn [đi bộ] khi trời mưa

This is very common in Vietnamese. Entire actions can function like nouns or clauses in English:

• Walking is healthy
• Going by bus is easier

Vietnamese does this without changing the verb into a special -ing form. The plain verb phrase can do the job.

Because Vietnamese adjectives can act directly as predicates.

English often needs to be:

• It is easy
• It is cheaper
• It is difficult

Vietnamese often does not:

• dễ = easy
• rẻ = cheap
• khó = difficult

So:

• đi xe buýt đến quán đó dễ hơn đi bộ
• not ... là dễ hơn... in normal usage

This is a very important difference from English. In many Vietnamese sentences, there is no separate verb meaning is before an adjective.

Because Vietnamese demonstratives usually come after the noun.

So:

• quán đó = that café / that shop / that place
• người đó = that person
• nhà đó = that house

This is different from English, where that comes first.

Also, quán is a general word often used for a small restaurant, café, eatery, or shop-like place, depending on context. So quán đó means that place of that type.

Because Vietnamese often leaves out repeated information when it is already understood.

The full version could be:

• đi xe buýt đến quán đó dễ hơn đi bộ đến quán đó khi trời mưa

But repeating đến quán đó is unnecessary, so it is omitted in the second half.

English does the same thing:

• Taking the bus there is easier than walking
• We do not need to say walking there if there is already obvious.

So this omission makes the sentence more natural and less repetitive.

It means when it rains or when the weather is rainy / when it is raining.

Breakdown:

• khi = when
• trời = sky / weather
• mưa = rain

The expression trời mưa is a very common Vietnamese way to say it is raining or the weather is rainy.

A learner may wonder why trời is there. It is simply part of a natural weather expression. Vietnamese often uses trời in weather statements:

• trời mưa = it rains / it is raining
• trời nắng = it is sunny
• trời lạnh = it is cold

In context, it most naturally applies to the comparison as a whole: when it rains, going by bus is easier than walking.

In other words, the rainy condition is the situation in which the speaker is making the comparison.

If you want to make that even clearer, you could move it to the front:

• Khi trời mưa, tôi nghĩ đi xe buýt đến quán đó dễ hơn đi bộ.

That version is also natural and may feel clearer to some learners.

Yes, but the meaning changes a little.

• khi trời mưa = when it rains
• lúc trời mưa = at the time when it rains / when it is raining
• nếu trời mưa = if it rains

So:

• khi gives a general when
• lúc sounds a bit more tied to a particular time or occasion
• nếu makes it conditional, like if

In your sentence, khi trời mưa is a good natural choice.

Yes, it is natural and understandable.

A few equally natural variations are:

• Tôi nghĩ đi xe buýt đến quán đó dễ hơn đi bộ khi trời mưa.
• Khi trời mưa, tôi nghĩ đi xe buýt đến quán đó dễ hơn đi bộ.
• Tôi nghĩ khi trời mưa, đi xe buýt đến quán đó dễ hơn đi bộ.

All of these are fine. The differences are mostly about emphasis and flow, not basic meaning.