صديقي خائف من الظلام.

In Arabic, present-tense to be is usually omitted.

So صديقي خائف literally looks like my friend afraid, but it means my friend is afraid.

This kind of sentence is called a nominal sentence. If you wanted a past-tense version, then Arabic would use a verb, for example:

كان صديقي خائفًا = My friend was afraid

صديقي means my friend.

It is made of:

• صديق = friend
• ـي = my

So:

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

The final ـي is a possessive suffix.

Because صديقي is already definite.

In Arabic, a noun becomes definite not only with الـ, but also when it is possessed.
So صديقي is definite because it means my friend, not just a friend.

Compare:

• صديق = a friend
• الصديق = the friend
• صديقي = my friend

Here, خائف functions like an adjective or state word, meaning afraid.

It comes from the verb خاف = to fear / to be afraid.

So:

• خاف = he was afraid / he feared
• خائف = afraid, fearful

In this sentence, خائف is the predicate describing صديقي.

Because it agrees with صديقي, which is masculine singular.

Arabic adjectives normally agree with the noun they describe in gender and number.

So here:

• صديقي خائف = My friend is afraid (male friend)

If the friend were female, you would say:

• صديقتي خائفة = My female friend is afraid

Arabic commonly uses من after words for fear in this kind of expression.

So:

• خائف من الظلام = afraid of the dark

Even though من often means from, here the natural English translation is of.

This is something you mostly just learn as a pattern:

• خائف من... = afraid of...
• خائف من الكلاب = afraid of dogs
• خائف من الامتحان = afraid of the exam

الظلام means the darkness or the dark.

So من الظلام can be understood as:

• of the dark
• of darkness

In natural English, afraid of the dark is the best translation.

It is pronounced aẓ-ẓalām, not al-ẓalām in careful pronunciation.

That is because ظ is a sun letter, so the ل of الـ assimilates to the next consonant.

So:

• written: الظلام
• pronounced: aẓ-ẓalām

Also, ظ is an emphatic sound, roughly like a heavy z for many learners.

In fully vocalized formal Arabic, you could write:

صديقي خائفٌ من الظلامِ

A quick breakdown:

• صديقي = the subject, with my attached; its case is not shown in the usual obvious way because of the ـي suffix
• خائفٌ = the predicate, nominative
• من = preposition
• الظلامِ = genitive after من

In normal unvoweled writing, these endings are usually not written, and in everyday reading they are often not pronounced fully.

Yes. You can also use the verb يخاف:

صديقي يخاف من الظلام = My friend is afraid of the dark / fears the dark

The difference is roughly:

• صديقي خائف من الظلام = describes his state or condition
• صديقي يخاف من الظلام = uses a verb, often sounding a bit more like a general fact or habitual fear

Both are natural, but the sentence you were given is a very common and clear nominal-sentence pattern.

This is a very typical Arabic nominal sentence pattern:

• subject/topic
  • description/comment

So:

• صديقي = the topic: my friend
• خائف من الظلام = what is being said about him: is afraid of the dark

This word order is very natural in MSA when you are describing someone or something.