صديقي ينزل الدرج من الطابق الثالث في الصباح.

صديقي means my friend.

The ending -ي is the attached possessive pronoun for my. So:

• صديق = a friend / friend
• صديقي = my friend

Arabic usually expresses possession by attaching a suffix like this, instead of using a separate word for my.

Because the verb ينزل already includes the subject he.

In Arabic, verbs often tell you who is doing the action.
ينزل means he goes down / he is going down / he comes down, depending on context.

So Arabic does not need هو unless you want emphasis or contrast.

• ينزل = he goes down
• هو ينزل = he goes down / he is going down (more emphatic)

ينزل is in the imperfect form, which is commonly used for:

• present actions: he is going down
• habitual actions: he goes down
• near future, depending on context: he will go down

In this sentence, because of في الصباح (in the morning), it most naturally sounds like a habitual action:

• My friend goes down the stairs from the third floor in the morning

It can mean any of those in English, depending on context.

The basic idea is to go down / descend. English may choose different natural translations:

• goes down the stairs
• comes down the stairs
• descends the stairs

In Modern Standard Arabic, ينزل is a very normal everyday verb for downward movement.

Arabic allows more than one normal word order.

This sentence uses subject + verb + object:

• صديقي = my friend
• ينزل = goes down
• الدرج = the stairs

Arabic also often allows verb + subject order:

• ينزل صديقي الدرج...

Both are grammatical in many contexts. Starting with صديقي can make the sentence feel more like it is introducing or focusing on my friend as the topic.

Because with this meaning, نزل can take a direct object in Arabic.

So:

• ينزل الدرج = he goes down the stairs

This is different from English, where we usually need down. Arabic does not need a separate word here.

The preposition من is used later with الطابق الثالث to show the starting point:

• من الطابق الثالث = from the third floor

So the structure is roughly:

• he goes down the stairs
• from the third floor

الدرج means the stairs.

Arabic often uses the definite article الـ when referring to something specific or understood from context. Here it sounds like a particular staircase in a building, not just stairs in general.

So:

• درج = stairs / a staircase
• الدرج = the stairs / the staircase

In a sentence like this, the definite form is very natural.

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

So:

• الطابق = the floor
• الثالث = the third

Together:

• الطابق الثالث = the third floor

Also, the adjective must match the noun in definiteness. Since الطابق is definite, الثالث is also definite.

This is a basic Arabic adjective pattern:

• البيت الكبير = the big house
• الطابق الثالث = the third floor

Yes, both are possible, but they work a little differently.

• في الصباح = in the morning
• صباحًا = in the morning / mornings, used adverbially

In your sentence, في الصباح is a very common and clear prepositional phrase.

So this sentence could also be expressed in another natural way, depending on style, but في الصباح is perfectly standard.

In fully vocalized MSA, the sentence could be written like this:

صديقي ينزلُ الدرجَ من الطابقِ الثالثِ في الصباحِ

Here is what is happening:

• صديقي = subject
  
  • in full grammatical analysis, the subject is nominative
• ينزلُ = verb, with final ـُ
• الدرجَ = direct object, so it is accusative
• من الطابقِ = after من, the noun is genitive
• الثالثِ = adjective matching الطابقِ, so also genitive
• في الصباحِ = after في, the noun is genitive

In everyday unvocalized Arabic writing, these endings are usually not written, so learners often have to infer them from grammar.

Yes, sometimes. In Arabic, verb agreement can behave differently depending on word order.

A useful general rule is:

• If the verb comes first, the verb is often singular even if the subject is plural.
• If the subject comes first, the verb usually agrees more fully with it.

In your sentence, the subject is singular masculine (صديقي), so the verb is ينزل either way. That means you do not see a difference here, but the rule becomes more noticeable with plural subjects.

Yes. That is exactly what من is doing here.

من means from, so it marks the starting point:

• من الطابق الثالث = from the third floor

So the sentence tells you both:

• the path/action: he goes down the stairs
• the starting location: from the third floor

Without من الطابق الثالث, the sentence would still be complete, but it would not specify where he starts.