Dormi tão bem naquele colchão que quase não ouvi a chuva durante a noite.

In Portuguese, the subject pronoun is often omitted because the verb ending already shows who the subject is.

• Dormi = I slept
• The ending -i tells you it is I

So Eu dormi is also correct, but eu is unnecessary unless you want emphasis or contrast, for example:

• Eu dormi, mas ele não = I slept, but he didn’t

Because bem is an adverb, and it modifies the verb dormi.

• bem = well
• bom / boa = good

So:

• Dormi bem = I slept well
• Foi uma noite boa = It was a good night

A native English speaker often has to remember that Portuguese usually uses bem where English uses well.

Tão ... que means so ... that.

In this sentence:

• Dormi tão bem ... que quase não ouvi a chuva
  = I slept so well ... that I hardly heard the rain

It is a result structure:

• tão bem = so well
• que = that

You will see the same pattern in many sentences:

• Estava tão cansado que adormeceu logo = He was so tired that he fell asleep immediately
• Falou tão baixo que ninguém ouviu = She spoke so quietly that nobody heard

Both are possible in Portuguese, but they do not mean exactly the same thing here.

• muito bem = very well
• tão bem ... que = so well ... that

Because the sentence continues with que quase não ouvi..., tão is the natural choice. It introduces a consequence.

Compare:

• Dormi muito bem naquele colchão. = I slept very well on that mattress.
• Dormi tão bem naquele colchão que quase não ouvi a chuva. = I slept so well on that mattress that I hardly heard the rain.

Naquele is a contraction of:

• em
  • aquele = naquele

Here it means in/on that.

This is important because Portuguese uses em in places where English often uses on:

• dormir num colchão = to sleep on a mattress
• literally, it is closer to sleep in/on a mattress

Also, naquele is masculine singular because colchão is masculine singular:

• aquele colchão
• naquele colchão

This is about Portuguese demonstratives: este, esse, aquele.

Very roughly:

• este = this
• esse = that (near the person spoken to, or already close in the context)
• aquele = that over there / that one more distant

In real usage, especially for learners, the differences can sometimes feel subtle. In this sentence, naquele colchão suggests that mattress as something a bit more removed in space, time, or memory.

A native English speaker should know that Portuguese often distinguishes more clearly than English between different kinds of that.

Because the sentence describes completed events from a specific night.

• dormi = I slept
• ouvi = I heard

This is the pretérito perfeito in Portuguese, used for finished actions.

It fits the idea of one completed situation:

• I slept well
• I hardly heard the rain
• it happened during that night

If you used the imperfect instead, the meaning would change:

• Dormia tão bem... could sound more habitual or descriptive
• ouvia would suggest an ongoing/background action rather than a completed event

So dormi and ouvi are the natural choices here.

It literally means I almost didn’t hear, but in natural English it is often best understood as:

• I hardly heard
• I barely heard

It suggests that the rain was heard only a little, or almost not at all.

So:

• quase não ouvi a chuva does not strongly mean total silence
• it means the speaker heard it very little

This is a very common Portuguese pattern:

• quase não dormi = I hardly slept
• quase não comi = I barely ate

Because quase não + verb is the normal way to say hardly / barely with a negative idea.

• quase não ouvi = I hardly heard
• quase não dormi = I hardly slept

If you move things around, the sentence may become awkward or change meaning. For example:

• não ouvi quase... usually sounds incomplete unless something follows, such as nada
• não ouvi quase nada = I heard almost nothing

So in this sentence, quase não ouvi is the standard and natural structure.

Portuguese often uses the definite article where English would not.

So:

• ouvi a chuva = I heard the rain

The article a makes the noun sound specific and natural in context: it is the rain that was falling that night.

English often says I heard rain or I heard the rain, but Portuguese more naturally uses the article here.

Durante a noite emphasizes the time span: something happened during the night, over the course of the night.

• durante a noite = during the night

By contrast:

• à noite usually means at night / at nighttime

So:

• Ouvi barulhos durante a noite = I heard noises during the night
• Trabalho à noite = I work at night

In your sentence, durante a noite fits well because it refers to the period when the rain was happening while the speaker was asleep.

Some parts can move, but the original order is very natural and idiomatic.

• Dormi tão bem naquele colchão que quase não ouvi a chuva durante a noite.

For example, durante a noite could also appear a little earlier or later depending on emphasis. But quase não ouvi should normally stay together.

Natural:

• ... que quase não ouvi a chuva durante a noite
• ... que durante a noite quase não ouvi a chuva

Less natural or potentially confusing:

• ... que não ouvi quase a chuva

So yes, Portuguese word order has some flexibility, but this sentence is already in a very good, natural order for everyday language.