يعمل صديقي خلال النهار.

In Arabic, possession is often shown with a suffix attached to the noun.

• صديق = friend
• ـي = my

So:

• صديقي = my friend

This is much more common than using a separate word for my the way English does.

This is a very common Arabic sentence pattern: verb + subject + other information.

So:

• يعمل = works
• صديقي = my friend
• خلال النهار = during the day

This gives:

• يعمل صديقي خلال النهار.

Arabic can also use صديقي يعمل خلال النهار. That is also correct, but starting with the verb is very natural in Modern Standard Arabic.

يعمل is in the present/imperfect form. In Arabic, this form can cover several English meanings depending on context, including:

• he works
• he is working
• he does work

In this sentence, because of خلال النهار (during the day), the meaning is most naturally something general or habitual:

• My friend works during the day.

The verb itself contains the subject information.

In يعمل:

• the beginning يـ usually marks he in this kind of verb form

So يعمل means he works.

That is why Arabic often does not need a separate word for he here. You could say هو يعمل, but in a normal sentence it is often unnecessary.

خلال means during or throughout.

So:

• خلال النهار = during the day

It is used to place an action within a span of time.

For example:

• خلال الأسبوع = during the week
• خلال الصيف = during the summer

الـ is the Arabic definite article, meaning the.

• نهار = daytime / a day
• النهار = the daytime / the day

In the phrase خلال النهار, Arabic normally uses the definite form, so the phrase means:

• during the day
• or more naturally in English, during the daytime

Because صديقي is already definite through possession.

A noun with a possessive ending like ـي (my) is automatically definite:

• صديق = a friend
• صديقي = my friend

So you do not say الصديقي.

This is a general rule: nouns with possessive suffixes do not take الـ.

Yes. In fully vowelled Classical/Modern Standard Arabic, the sentence would be:

• يَعْمَلُ صَدِيقِي خِلَالَ النَّهَارِ

Here is what is happening:

• يعملُ: the verb has -u
• صديقي: the subject is nominative, but with ـي attached, the case is not shown in the simple way learners may expect
• خلالَ: often analyzed here as accusative as an adverbial/time expression
• النهارِ: genitive after خلال

In normal everyday writing, these short vowels are usually omitted.

It is pronounced approximately ṣa-dī-qī.

A few important points:

• ص is an emphatic s, heavier than English s
• دي gives a long dī
• قي gives qī, where ق is a deep q sound, not an English k

So the whole word is:

• صديقي = ṣadīqī

If you are a beginner, a rough pronunciation guide is:

• sa-DEE-qee

But try to learn the proper sounds ص and ق as early as possible.

Yes, in many contexts you can, and the meaning will be similar.

• خلال النهار = during the day
• في النهار = in the daytime / during the day

However, خلال often sounds a little more specifically like during the course of a period, while في is the more basic and general preposition in.

Both are natural, but خلال النهار is a very good formal MSA expression.

Yes. That is also correct.

Arabic allows both:

• يعمل صديقي خلال النهار
• صديقي يعمل خلال النهار

The difference is mainly one of style and emphasis:

• verb-first is very common and neutral in MSA
• subject-first can sound slightly more topic-focused, like As for my friend, he works during the day

Both are useful and natural to know.