هو يحجز موعدا اليوم.

In Arabic, the verb يحجز already includes the meaning he books / he is booking, so the sentence could simply be:

يحجز موعدًا اليوم.

Adding هو is usually for one of these reasons:

• emphasis: He is booking an appointment today.
• clarity: to make the subject extra clear
• contrast: He is booking an appointment today, not someone else

So هو is not strictly necessary, but it is perfectly natural.

يحجز is in the present tense / imperfect.

In MSA, this form can mean different things depending on context, including:

• he books
• he is booking
• he will book

In this sentence, because of اليوم (today), the most natural meaning is something like:

• He is booking an appointment today or
• He books an appointment today

Arabic does not separate simple present and present progressive as strictly as English does.

The dictionary form is حَجَزَ.

That is the past tense form, and it means to reserve, book, detain, block, depending on context.

From that verb:

• حَجَزَ = he booked / reserved
• يَحْجُزُ = he books / is booking

Its root is ح-ج-ز.

Because موعدًا is the direct object of the verb يحجز.

He is booking what?
→ an appointment

In MSA, direct objects usually take the accusative case, and when the noun is indefinite, that often appears as tanwīn fatḥ: -ًا / -an.

So:

• موعدٌ = an appointment (subject form / nominative)
• موعدًا = an appointment (object form / accusative)

That is why you see موعدًا here.

This is a spelling rule for tanwīn fatḥ.

Many Arabic nouns that end with -an in the accusative are written with:

• the tanwīn fatḥ sign ً
• plus an extra written alif

So:

• موعدًا

This extra alif is mostly an orthographic spelling feature. It helps show the -an ending in writing.

Because اليوم is definite, thanks to ال (the).

Tanwīn is generally used with indefinite nouns, not definite ones.

So:

• موعدًا = an appointment → indefinite
• اليوم = the day / today → definite

Also, in this sentence اليوم functions as an adverb of time: today.

In fully vowelled MSA, you may see it as اليومَ, with a final fatḥa because it is functioning adverbially.

موعد commonly means:

• appointment
• meeting time
• date (in the sense of a scheduled time)

So its exact translation depends on context.

For example:

• لدي موعد مع الطبيب = I have a doctor’s appointment
• موعد الاجتماع = the time of the meeting

In this sentence, appointment is the natural meaning.

Yes. Arabic word order is more flexible than English.

The sentence you have is:

هو يحجز موعدًا اليوم.

This is a subject + verb + object + time type structure.

But Arabic could also use other orders, for example:

• يحجز هو موعدًا اليوم
• اليوم يحجز موعدًا
• يحجز موعدًا اليوم

The meaning stays similar, but the focus changes.

For learners, the original sentence is easy and natural, especially because it clearly shows the subject first.

Not always.

In fully correct formal MSA, the case endings are:

• يَحْجُزُ
• مَوْعِدًا
• اليَوْمَ

But in everyday speech, news reading aside, and in less formal pronunciation, many speakers do not fully pronounce all case endings.

So you may hear something closer to:

• هو يحجز موعد اليوم or
• هو يحجز موعدًا اليوم

depending on the level of formality and the speaker.

In writing, especially educational or grammatical contexts, the full case analysis still matters.

Arabic has no separate word for the English articles a / an.

Instead, indefiniteness is usually shown by the noun itself, often with tanwīn.

So:

• موعد / موعدًا = an appointment
• الموعد = the appointment

That means موعدًا already carries the idea of an appointment, without needing a separate word for an.

A fully vowelled version would usually be:

هُوَ يَحْجُزُ مَوْعِدًا اليَوْمَ.

Breakdown:

• هُوَ = he
• يَحْجُزُ = he books / is booking
• مَوْعِدًا = an appointment
• اليَوْمَ = today

This is useful for learners because it shows:

• the verb ending -u
• the object ending -an
• the adverbial ending -a

Not in normal English translation.

Although the root meaning of حجز involves booking / reserving, and موعد is related to a set time, the natural meaning of يحجز موعدًا is:

• he books an appointment
• he makes an appointment

So while a very literal breakdown might suggest something like reserves a time, that would not be the normal way to translate it.