صديقي يتصل بي بعد الدرس.

Because صديق means friend, and the suffix ـي means my.

So:

• صديق = friend
• صديقي = my friend

This suffix is attached directly to the noun, so Arabic does not need a separate word for my here.

Yes, يتصل comes from اتصل, which commonly means to contact or to call, especially by phone.

In Modern Standard Arabic, اتصل بـ is the usual pattern for to call/contact someone.

So:

• اتصل بي = he called me / he contacted me
• يتصل بي = he calls me / he is calling me / he will call me, depending on context

In a sentence like this, many learners will naturally understand it as My friend calls me after class or My friend will call me after class.

Because this verb normally takes the preposition بـ before the person being contacted.

So Arabic says:

• يتصل بي = literally something like he contacts/calls with me
• but in natural English, this simply means he calls me

This is just a verb pattern you need to learn:

• اتصل بـ = to contact/call

Many Arabic verbs work with a preposition where English uses a direct object.

بي is made of:

• بـ = with / by / to (depending on the verb)
• ـي = me / my

Together, بي here means me after the verb يتصل.

It is pronounced bī.

So:

• يتصل بي = he calls me

يتصل is in the imperfect tense.

In Arabic, the imperfect can express different English ideas depending on context, including:

• he calls
• he is calling
• he will call

In this sentence, because of بعد الدرس (after class / after the lesson), the meaning is usually understood as either a habitual action or a future one:

• My friend calls me after class
• My friend will call me after class

In the imperfect tense, يـ often marks third person masculine singular.

So يتصل means:

• he calls / he is calling

If the subject were feminine, you would usually get تتصل:

• صديقتي تتصل بي = My female friend calls me

So the يـ helps show that the subject is he.

الدرس means the lesson or the class.

So:

• بعد الدرس = after the lesson / after class

Arabic often uses the definite article in places where English may sound more general. In context, it usually means a specific lesson or class already understood by speaker and listener.

If you said بعد درس, it would sound more like:

• after a lesson
• after some lesson

So بعد الدرس is the more natural choice here if a particular class is meant.

Arabic allows more than one natural word order.

This sentence begins with the subject:

• صديقي يتصل بي بعد الدرس

That gives a sense like:

• My friend calls me after class

You could also say:

• يتصل بي صديقي بعد الدرس

That is also grammatical. Verb-first order is very common in Arabic.

Very roughly:

• صديقي يتصل... puts a little more focus on my friend
• يتصل صديقي... feels more like a straightforward verbal statement

Both are correct.

A careful pronunciation is:

ṣadīqī yattaṣilu bī baʿda d-darsi

A few useful notes:

• صديقي = ṣa-dī-qī
• يتصل is pronounced yattaṣilu when fully vocalized
• بي = bī
• بعد = baʿda
• الدرس is pronounced ad-dars in connected speech, because د is a sun letter, so the l of ال is not pronounced

So بعد الدرس sounds like:

• baʿda d-darsi

Yes. When fully vocalized, it is:

يَتَّصِلُ

So the ت is doubled in pronunciation.

In normal Arabic writing, short vowels and shadda are often omitted, so you usually just see:

يتصل

But a native reader knows from the verb pattern that it is read as yattaṣilu.

If fully vocalized, the sentence is:

صديقي يَتَّصِلُ بي بعدَ الدرسِ

A few notes:

• يَتَّصِلُ ends in ـُ because it is an indicative imperfect verb
• بعدَ is usually pronounced with fatḥa
• الدرسِ has kasra because it comes after بعد in an iḍāfa-like relationship, so it is genitive

As for صديقي, the case is not clearly shown at the end in ordinary writing because of the ـي suffix (my). Learners often just memorize the form as ṣadīqī.

It can mean either, depending on context.

• الدرس can be the lesson
• It can also be understood as the class

So بعد الدرس may be translated naturally as:

• after the lesson
• after class

In many learning contexts, after class sounds the most natural in English, even though the Arabic literally contains the lesson.