صاحبتي بتكتب رسالة بعد الدرس عشان الامتحان.

Because it is made of:

• صاحبة = female friend / female companion
• -ي = my

When a feminine noun ending in ـة takes a suffix, that ending usually shows up as ت. So:

• صاحبة
  • ي → صاحبتي

So صاحبتي literally means my female friend. The ت is not a separate word; it is part of how the feminine form connects to the possessive ending.

It can mean either, depending on context.

In everyday Egyptian Arabic, صاحبتي is commonly used for:

• my female friend
• my girlfriend

The situation and tone tell you which meaning is intended. If someone wants to be more formal or more clearly mean female friend, they might use صديقتي instead, though that sounds more formal or MSA-like than everyday Egyptian speech.

Because the verb already shows the subject.

بتكتب means she writes / she is writing (and can also be you write for a singular feminine you). In this sentence, صاحبتي tells you who is doing the action, so adding هي is unnecessary.

Arabic often does this: the subject pronoun is left out unless it is needed for emphasis or contrast.

In Egyptian Arabic, بـ usually marks the normal present/non-past form.

So:

• تكتب by itself is not the usual everyday present form in Egyptian
• بتكتب is the common spoken form meaning she writes or she is writing

This بـ is a very important feature of Egyptian Arabic verbs in the present tense.

It can mean both.

In Egyptian Arabic, the same present form often covers:

• she writes
• she is writing

Context tells you which one is meant. If you want to make right now especially clear, speakers often add a time word like دلوقتي.

So بتكتب does not force a difference the way English does.

Yes.

رسالة is a general word for a message in writing, so it can mean:

• letter
• message
• sometimes even note

The exact meaning depends on context. In modern speech, it can also refer to things like messages, not just old-fashioned letters.

Because Arabic does not have an indefinite article like English a/an.

A bare noun can mean a or an:

• رسالة = a letter / a message
• الرسالة = the letter / the message

So the lack of الـ makes رسالة indefinite.

بعد means after, and it is followed by the noun phrase الدرس = the lesson / the class.

So:

• بعد الدرس = after the lesson / after class

As for pronunciation: الدرس is spelled with الـ, but the ل of الـ is not pronounced normally before certain letters called sun letters. The letter د is one of them.

So الدرس is pronounced roughly like:

• id-dars or ed-dars

with the d sound doubled.

عشان is a very common Egyptian word that can mean things like:

• for
• because of
• for the sake of
• so that

In this sentence, it most naturally gives the reason or purpose:

• عشان الامتحان = for the exam / because of the exam

So the idea is that the writing is connected to the exam in some way.

Because the sentence is probably talking about a specific, known exam.

• الامتحان = the exam
• امتحان = an exam / an exam in general

Arabic often uses the definite article when both speaker and listener know which thing is being talked about. In a school context, the exam is often understood naturally.

Because عشان is the more natural way to express reason/purpose here.

• عشان الامتحان = for the exam / because of the exam
• للـامتحان would sound more like for the exam in a very literal prepositional sense, and in this sentence it is less natural

So عشان is doing the job of explaining why she is writing the message.

Yes, very natural.

The sentence is:

• صاحبتي = subject
• بتكتب = verb
• رسالة = object
• بعد الدرس = time phrase
• عشان الامتحان = reason/purpose phrase

A subject-first order like this is extremely common in spoken Egyptian Arabic. It sounds normal and conversational.