بعد الغداء أنام على السرير ساعة، ثم أجلس على الكرسي قرب النافذة.

Because the verbs already show the subject.

• أنام = I sleep
• أجلس = I sit

In Modern Standard Arabic, the verb form often tells you who is doing the action, so an independent pronoun like أنا is usually unnecessary unless you want emphasis.

So:

• أنام by itself already means I sleep
• أنا أنام would mean I sleep, but with extra emphasis on I

بعد الغداء means after lunch, and it sets the time for what follows.

Arabic often begins a sentence with a time expression, especially in descriptions of habits or routines. This is very natural in MSA.

So the structure is:

• بعد الغداء = after lunch
• أنام... = I sleep...

Literally: After lunch, I sleep...

You could also say the same idea with the time expression later in the sentence, but starting with it is common and sounds natural.

They are in the present/imperfect form, but here they express a habitual action.

So in this sentence:

• أنام does not necessarily mean I am sleeping right now
• أجلس does not necessarily mean I am sitting right now

Instead, they mean something like:

• I sleep
• I sit

in the sense of a routine or usual action.

This is very common in Arabic: the present form can describe present actions, repeated actions, or general habits, depending on context.

Because Arabic commonly uses على (on) with things like a bed or a chair.

• على السرير = on the bed
• على الكرسي = on the chair

For an English speaker, on the chair feels normal, but on the bed may sound different because English often says in bed. Arabic can express this differently, and على السرير is perfectly natural for the physical idea of being on top of the bed.

So this is a case where Arabic preposition use does not match English exactly.

In Arabic, a duration of time can often be expressed directly without a separate word for for.

So:

• أنام ساعة = I sleep for an hour

Here ساعة means an hour, and it functions as an expression of duration.

This is a very common pattern in Arabic:

• انتظرت ساعة = I waited for an hour
• درست يومين = I studied for two days

So the idea of for is understood from the structure.

ثم means then and shows sequence, usually with a sense that one action happens after the other.

So:

• ثم أجلس = then I sit

By contrast, و simply means and and does not clearly emphasize order in the same way.

Compare:

• نمت ثم جلست = I slept, then I sat
• نمت وجلست = I slept and sat

Both can work, but ثم is better when you want to show a clear next step in a sequence.

قرب النافذة means near the window.

In traditional grammar, قرب is often treated as a noun or adverb-like word meaning nearness/proximity, and it can function much like a preposition in sentences like this.

So:

• قرب النافذة = near the window

You may also see another common way to say near:

• قريب من النافذة = near the window

Both are correct, but the structure in your sentence is shorter and very common.

Because they are definite nouns:

• الغداء = the lunch / lunch
• السرير = the bed
• الكرسي = the chair
• النافذة = the window

Arabic uses the definite article الـ more often than English in some contexts.

For example, الغداء literally looks like the lunch, but in English we usually just say lunch. Arabic often uses the definite form for familiar daily things like meals.

Likewise, السرير, الكرسي, and النافذة can be definite because the speaker has specific objects in mind, or because the context makes them identifiable.

With full vowels, the sentence would typically be:

بعدَ الغداءِ أنامُ على السريرِ ساعةً، ثمَّ أجلسُ على الكرسيِّ قربَ النافذةِ.

A learner might notice these endings:

• بعدَ: accusative as an adverbial time expression, and it is also the first part of an iḍāfa
• الغداءِ: genitive because it is the second part of that iḍāfa
• أنامُ: indicative present
• على السريرِ: السريرِ is genitive after the preposition على
• ساعةً: accusative because it expresses duration
• أجلسُ: indicative present
• على الكرسيِّ: الكرسيِّ is genitive after على
• قربَ النافذةِ: قربَ is in the accusative in this adverbial use, and النافذةِ is genitive after it in the construct relationship

In normal everyday writing, these short vowels are usually omitted.

Yes, Arabic word order is fairly flexible, though some versions sound more natural than others.

The given sentence:

بعد الغداء أنام على السرير ساعة، ثم أجلس على الكرسي قرب النافذة.

is very natural.

You could also say something like:

أنام بعد الغداء على السرير ساعة، ثم أجلس على الكرسي قرب النافذة.

This still makes sense, but the original version is smoother if you want to start by setting the time.

So the answer is:

• Yes, the order can change
• But the original order is a normal and natural way to describe a routine

Because جلس / يجلس can often mean both sit and sit down, depending on context.

In English, we sometimes distinguish:

• sit = be seated
• sit down = move into a seated position

Arabic does not always make that distinction as sharply in everyday use. In a sequence like this, ثم أجلس can naturally mean then I sit or then I sit down, depending on the intended meaning.

If someone wanted to emphasize the act of becoming seated, context would usually make it clear.

Arabic can use both, but they are not always exactly the same in feeling.

• على السرير = on the bed
• في السرير = in bed

In your sentence, على السرير focuses more on the physical location: being on the bed.

في السرير is also common in Arabic, especially when talking about being in bed in a more general or idiomatic sense, similar to English in bed.

So the sentence as written is correct, but a learner should know that Arabic may use either expression depending on nuance and style.