Ela está tão cansada que precisa de se deitar assim que chegar a casa.

Because estar is used for a temporary state or condition, and being tired is normally temporary.

• Ela está cansada = she is tired
• Ela é cansada would sound odd here and would not normally mean she is tired. It could suggest something more like she is tiring/annoying or just sound unnatural.

So for physical or emotional states such as cansada, doente, feliz, nervosa, Portuguese usually uses estar.

Because adjectives in Portuguese usually agree with the person they describe.

Here the subject is ela, so the adjective must be feminine singular:

• ela está cansada
• ele está cansado

So cansada matches ela.

Tão ... que means so ... that.

So:

• tão cansada que precisa de se deitar = so tired that she needs to lie down

This is a fixed pattern:

• tão + adjective/adverb + que

Examples:

• tão tarde que... = so late that...
• tão depressa que... = so quickly that...

By contrast, muito usually means very:

• muito cansada = very tired

So:

• muito cansada = very tired
• tão cansada que... = so tired that...

In European Portuguese, precisar normally takes de.

That means you get:

• precisar de ajuda = to need help
• precisar de descansar = to need to rest
• precisar de se deitar = to need to lie down

English does not translate this de directly. It is just part of how the verb works in Portuguese.

This is especially important for learners because in some varieties of Brazilian Portuguese, people often omit de before an infinitive in everyday speech. In Portugal, precisar de is the standard pattern.

The dictionary form is deitar-se, a reflexive/pronominal verb.

Here it means to lie down or sometimes to go to bed, depending on context.

The se is important because:

• deitar on its own often means to lay/put something down
• deitar-se means to lie oneself down

So in this sentence, since she is the one lying down, Portuguese uses the pronominal form:

• precisa de se deitar

Because in European Portuguese, when a reflexive/object pronoun comes with an infinitive after a preposition, it commonly appears before the infinitive.

So you often get patterns like:

• antes de me levantar = before getting up
• para se sentar = to sit down
• de se deitar = to lie down

The dictionary form is still deitar-se, but inside a structure like de + infinitive, European Portuguese commonly uses se deitar.

Assim que means as soon as.

So:

• assim que chegar a casa = as soon as she gets home

It introduces an action that happens immediately after another one.

Other similar expressions are:

• logo que
• quando (sometimes, depending on context)

But assim que strongly suggests as soon as.

Because Portuguese uses the future subjunctive after certain time expressions when the action is in the future.

Assim que is one of those triggers.

So:

• assim que chegar a casa = as soon as she gets home
• quando chegar = when she arrives
• logo que terminar = as soon as it finishes

In English, we usually use the present form after when/as soon as:

• as soon as she gets home

But in Portuguese, the form here is the future subjunctive: chegar.

A useful contrast:

• Assim que chega a casa, deita-se. = As soon as she gets home, she lies down.
  (habit/routine)
• Assim que chegar a casa, vai deitar-se. = As soon as she gets home, she will lie down.
  (future)

Because casa without an article often means home.

So:

• chegar a casa = to get home
• ir a casa = to go home
• voltar a casa = to return home

If you say à casa, that sounds more like to the house as a physical building, not simply home.

And em casa means at home, not to home.

So:

• chegar a casa = get home
• estar em casa = be at home

It can mean either, depending on context.

• deitar-se literally means to lie down
• In many contexts, especially at night, it can also mean to go to bed

In this sentence, since she is extremely tired and needs to do it as soon as she gets home, the most natural sense is probably lie down or go and lie down immediately.

But if the wider context is nighttime or sleep, go to bed could also be a valid interpretation.

No. That would be too literal and unnatural in English.

Several parts do not map word-for-word:

• precisa de does not become needs of
• se deitar does not become lie herself down in normal English
• chegar a casa is naturally get home, not always arrive to home

A natural English translation would be something like:

• She is so tired that she needs to lie down as soon as she gets home.

This is a good example of why Portuguese should be understood in chunks:

• tão ... que = so ... that
• precisar de + infinitive = need to ...
• assim que = as soon as
• chegar a casa = get home