الولد والبنت كانوا قاعدين جنب الشباك وبيتفرجوا على فيلم.

Because the subject is the boy and the girl, which counts as more than one person, so the verb is plural.

• كان = was
• كانوا = were

So كانوا قاعدين means they were sitting.

In this sentence, كانوا sets the whole scene in the past.

In Arabic, a mixed group of males and females normally takes masculine plural agreement.

So even though it is a boy + a girl, Arabic uses:

• كانوا
• قاعدين
• بيتفرجوا

rather than feminine plural forms.

This is a very common rule in Arabic, including Egyptian Arabic.

Yes, Arabic has a dual, but in Egyptian Arabic, with a subject like X and Y, speakers normally use plural agreement, not a special dual verb form.

So الولد والبنت كانوا... is completely natural in Egyptian Arabic.

The dual is more noticeable in certain nouns, especially in more formal Arabic, but in everyday Egyptian speech, plural agreement is the normal choice here.

قاعدين is the plural form of قاعد, which literally means sitting or seated.

So:

• قاعد = sitting / seated
• قاعدين = sitting / seated (plural)

With كانوا, it gives:

• كانوا قاعدين = they were sitting

This is a very common Egyptian way to describe someone’s state or position.

Because Egyptian Arabic very often uses كان + active participle to describe a past state.

So instead of something that would feel like they sat, the sentence uses they were sitting.

That makes sense here because the sentence is describing a scene in progress:

• they were sitting by the window
• and they were watching a movie

It is more like background description than a single completed action.

In Egyptian Arabic, بيـ is a common marker on the present/imperfect verb.

So:

• يتفرجوا = they watch / watch
• بيتفرجوا = they are watching / they watch

In this sentence, because كانوا already put things in the past, وبيتفرجوا is understood as and they were watching.

So the whole sentence has a past continuous feel.

Because Arabic does not need to repeat it here.

Once كانوا has established the past background, it can carry over to the next linked action:

• كانوا قاعدين... وبيتفرجوا...

This is understood as:

• they were sitting ... and watching ...

You could repeat كانوا in some contexts, but it is not necessary here, and leaving it out sounds natural.

Because the verb يتفرج على in Egyptian Arabic means to watch / look at.

So the preposition is part of the verb pattern:

• يتفرج على شيء = to watch something

That is why the sentence says:

• بيتفرجوا على فيلم = they were watching a movie

English says watch a movie with no preposition, but Egyptian Arabic uses على.

جنب means beside, next to, or by.

So:

• جنب الشباك = by the window / next to the window

In Egyptian Arabic, جنب can work very naturally without another preposition before it.

Because الشباك is the normal everyday Egyptian Arabic word for window.

• شباك = window, in colloquial Egyptian
• نافذة = window, but more formal / Standard Arabic

So الشباك sounds natural in spoken Egyptian.

Because it means a movie, not the movie.

• فيلم = a movie
• الفيلم = the movie

In Egyptian Arabic, indefinite nouns are often just written as the bare word, with no extra ending.

So على فيلم means a movie.

In Egyptian Arabic, الـ is usually pronounced more like il- or el- in everyday speech.

So:

• الولد sounds like el-walad or il-walad
• الشباك sounds like esh-shubbāk or ish-shubbāk

Why the difference?

Because ش is a sun letter, so the l sound of الـ is absorbed:

• الشباك → esh-shubbāk

But و in الولد is not a sun letter, so the l stays:

• الولد → el-walad

A few things:

1. بيتفرجوا

   • This is a very Egyptian colloquial verb form.
   • In Standard Arabic, you would more likely see a different verb such as يشاهدون.

2. الشباك

   • This is the common Egyptian colloquial word for window.
   • Standard Arabic would more often use النافذة.

3. The overall structure

   • كانوا قاعدين ... وبيتفرجوا ...
   • This sounds very natural in spoken Egyptian.

So the sentence is clearly colloquial Egyptian, not formal Standard Arabic.