صديقي خائف اليوم.

Because Arabic often uses nominal sentences in the present tense, and in those sentences there is usually no separate word for is/am/are.

So:

• صديقي خائف اليوم literally looks like my friend afraid today
• but it naturally means My friend is afraid today

If you wanted a past tense, Arabic would use a verb, for example كان for was.

It is a nominal sentence in Arabic.

A basic nominal sentence often has:

• a subject/topic called المبتدأ
• a predicate/comment called الخبر

In this sentence:

• صديقي = the subject/topic
• خائف = the predicate
• اليوم = an adverbial time expression, today

So the structure is:

• صديقي — my friend
• خائف — afraid
• اليوم — today

صديقي means my friend.

It is made from:

• صديق = friend
• ـي = my

So:

• صديق → friend
• صديقي → my friend

The ـي at the end is a possessive suffix attached directly to the noun.

In Arabic, possessive words like my, your, his, her are often expressed with suffixes attached to the noun.

Examples:

• صديقي = my friend
• صديقك = your friend
• صديقه = his friend
• صديقتها = her friend

So instead of using a separate word like English my, Arabic usually adds something to the end of the noun.

Because خائف is not the verb to fear. It is an active participle that functions here like an adjective or predicate meaning afraid or fearful.

So in this sentence, خائف describes the state of the friend:

• صديقي خائف = My friend is afraid

It is not saying my friend fears something.
That would require a different structure with a verb.

It is adjective-like in meaning, but grammatically here it is the predicate of the sentence, not just an adjective attached inside a noun phrase.

Compare:

• صديقي خائف = My friend is afraid
  Here خائف is the predicate.

Versus something like:

• صديقي الخائف = my afraid friend / my fearful friend
  Here الخائف is an adjective modifying friend inside the noun phrase.

So in your sentence, خائف is better understood as the part that tells us something about صديقي.

Because predicates in simple nominal sentences are often indefinite.

So:

• صديقي is definite because it means my friend
• خائف is indefinite and works as the predicate: afraid

This is very common in Arabic:

• الولد طويل = The boy is tall
• البيت كبير = The house is big
• صديقي خائف = My friend is afraid

If you added الـ and said الخائف, the meaning and structure would shift.

In normal Arabic writing, short vowels and case endings are usually not written.

A fully vowelled formal version could be:

• صديقي خائفٌ اليومَ

In careful grammar terms:

• صديقي is the subject
• خائفٌ is nominative as the predicate
• اليومَ is often treated as an adverbial accusative of time

But in everyday printed Arabic, these endings are usually omitted, so you commonly just see:

• صديقي خائف اليوم

A helpful pronunciation is:

• ṣadīqī khā'if al-yawm

Very roughly for an English speaker:

• ṣa = an emphatic s
• dī = like dee
• qī = kee but with a deeper q sound from the back of the mouth
• khā'if = starts with kh, like the ch in German Bach or Scottish loch
• al-yawm = al-yowm

A rough full approximation:

• sa-DEE-qee KHAa-if al-YOWM

Not perfect, but useful as a starting point.

Usually these:

• ص in صديقي
  This is an emphatic s, deeper and heavier than normal س.

• ق in صديق
  In MSA this is qāf, pronounced at the back of the mouth, not like ordinary English k.

• خ in خائف
  This is the kh sound, like the ch in Bach/loch.

Also, خائف contains a hamza sound in the middle: khā'if.
That little break matters.

You would usually say:

• صديقتي خائفة اليوم

Changes:

• صديقي → صديقتي = my female friend
• خائف → خائفة = afraid, feminine form

So the predicate agrees in gender with the subject.

Yes, in this kind of sentence the predicate normally agrees with the subject in gender and number.

Since صديقي refers to one masculine person, the predicate is:

• خائف = masculine singular

If the subject were feminine singular:

• صديقتي خائفة

If the subject were plural, the form would change accordingly.

Yes. Arabic word order is somewhat flexible, especially with time expressions like اليوم.

You can say:

• صديقي خائف اليوم
• اليوم صديقي خائف
• صديقي اليوم خائف

These can all work, though the emphasis may shift slightly.

The original version is very natural and straightforward.

Because Arabic often uses time expressions directly without a preposition where English uses one.

So:

• اليوم = today
• غدًا = tomorrow
• أمس = yesterday

You can often place them directly in the sentence:

• أنا مشغول اليوم = I am busy today
• صديقي خائف اليوم = My friend is afraid today

So this is normal Arabic usage.

Yes, but it is usually less neutral in a simple sentence.

The most natural basic version is:

• صديقي خائف اليوم

Adding هو can create extra emphasis or a slightly different style:

• صديقي هو خائف اليوم

In beginner Arabic, it is usually best to learn the simpler standard pattern first:
subject + predicate, without an extra pronoun.

No, but it is one of the most common standard words in MSA.

• صديق = male friend / friend
• صديقة = female friend

In Modern Standard Arabic, this is the normal word to learn first.
Different spoken dialects may use other common words too, but صديق is a good standard choice.

Yes, roots are very useful in Arabic.

• صديق comes from the root ص د ق
  • connected with ideas of truthfulness and sincerity
• خائف comes from the root خ و ف
  • connected with fear

Knowing roots helps you connect related words. For example, from خ و ف you also get words related to fear, such as:

• خوف = fear

This is one of the most helpful patterns in Arabic vocabulary learning.