يتكلم المدرب الشخصي بشأن وضعية الجسم.

Because this is a normal verbal sentence in Arabic. In Modern Standard Arabic, it is very common to start with the verb, then give the subject, then the rest of the sentence.

So:

• يتكلم = speaks / is speaking
• المدرب الشخصي = the personal trainer

This verb-first order is very natural in MSA. You could also say:

المدرب الشخصي يتكلم بشأن وضعية الجسم

That is also correct, but it feels more like a subject-first sentence.

يتكلم is the imperfect form of the verb تكلّم.

Here it can mean:

• speaks
• is speaking
• talks

Arabic imperfect verbs often cover both the simple present and the present progressive, so the exact English translation depends on context.

Grammatically, يتكلم is:

• 3rd person
• masculine
• singular

So it matches المدرب, which is one male trainer.

Because المدرب الشخصي refers to one person.

The two words are:

• المدرب = the trainer
• الشخصي = personal

So الشخصي is just an adjective, not a second person or second subject. The whole phrase still means one personal trainer, so the verb stays singular:

يتكلم = he speaks / is speaking

Because in Arabic, adjectives normally come after the noun they describe.

So:

• المدرب = the trainer
• الشخصي = personal

Together:

• المدرب الشخصي = the personal trainer

This is the opposite of English word order, where the adjective usually comes first.

Also, the adjective must agree with the noun in:

• definiteness
• gender
• number
• case

That is why both words have الـ here.

بشأن means regarding, concerning, or about.

So:

• بشأن وضعية الجسم = about body posture / regarding body posture

This expression is a bit more formal than some other ways of saying about in Arabic.

A very common alternative is عن:

• يتكلم عن وضعية الجسم
• يتحدث عن وضعية الجسم

Those are also very natural.
Using بشأن gives the sentence a slightly more formal or written tone.

Because this is an iḍāfa construction, often called a genitive construction or noun-of-noun structure.

It works like this:

• وضعية = posture / position
• الجسم = the body

Together:

• وضعية الجسم = the posture of the body / body posture

In an iḍāfa construction:

1. the first noun usually does not take الـ
2. the second noun can make the whole phrase definite

So وضعية الجسم is correct, while الوضعية الجسم is not the normal MSA form here.

Because in an iḍāfa construction, the definiteness of the whole phrase is determined by the last noun.

Here:

• الجسم = the body (definite)

So the whole phrase becomes definite:

• وضعية الجسم = the body’s posture / body posture

This is a very important Arabic pattern.
The first noun in iḍāfa often looks indefinite by itself, but the whole phrase can still be definite.

Because ة (taa marbuuTa) is often pronounced differently depending on context.

When a word is said by itself or in pause, it often sounds like -a or -ah:

• وضعية ≈ waḍʿiyya

But when it is connected to another word, especially in an iḍāfa phrase or with case endings, it is pronounced as t:

• وضعية الجسم = waḍʿiyyat al-jism
• with full case marking here: waḍʿiyyati al-jismi

So yes, that ة becomes a real t sound in connected speech.

Because ش is a sun letter.

When الـ comes before a sun letter, the l sound of الـ is not pronounced. Instead, the following consonant is doubled.

So:

• written: الشخصي
• pronounced: ash-shakhṣiyy...

The spelling does not change, but the pronunciation does.

This same thing happens with many other letters, such as:

• التدريب → at-tadrīb
• الرجل → ar-rajul
• السماء → as-samāʾ

If you fully vowel the sentence, it is:

يَتَكَلَّمُ المُدَرِّبُ الشَّخْصِيُّ بِشَأْنِ وَضْعِيَّةِ الجِسْمِ

Here is why:

• يَتَكَلَّمُ → verb in the indicative
• المُدَرِّبُ → nominative, because it is the subject
• الشَّخْصِيُّ → nominative, because it agrees with المُدَرِّبُ
• بِشَأْنِ → genitive after bi-
• وَضْعِيَّةِ → genitive in the construction after بشأن
• الجِسْمِ → genitive as the second term of the iḍāfa

In normal unvowelled writing, you usually will not see these endings, but they are still part of the grammar.

Yes. A very natural alternative would be:

يتحدث المدرب الشخصي عن وضعية الجسم

or

يتكلم المدرب الشخصي عن وضعية الجسم

These versions are a bit more everyday because عن is a very common way to say about.

So the sentence you were given is perfectly correct, but بشأن sounds slightly more formal or written than عن.