هو يعتقد أن الموعد اليوم.

In Arabic, the verb to be is usually omitted in the present tense.

So:

• الموعد اليوم literally looks like the appointment today
• but it means the appointment is today

This is called a nominal sentence. Arabic often says the equivalent of X today / X important / X here without adding a present-tense is or are.

If you wanted the past or future, Arabic would usually show that more explicitly, for example with كان for was.

Here أن means that and introduces a subordinate clause:

• هو يعتقد = he thinks / believes
• أن الموعد اليوم = that the appointment is today

So the whole structure is:

• هو يعتقد + أن + clause

A very important note: in fully vocalized Arabic, this is أَنَّ (with a doubled n sound), not the other أَنْ used before verbs.

This is one of the most common learner questions, because both are usually written as أن without vowels.

They are different words:

• أَنَّ introduces a noun clause, often translated as that
  • as in this sentence: هو يعتقد أن الموعد اليوم
• أَنْ is usually followed by a verb and often corresponds to to or a clause like that he should...

Compare:

• أعتقد أنَّ الموعدَ اليومَ = I think that the appointment is today
• أريد أن أذهبَ = I want to go

So in your sentence, it must be أَنَّ, because what follows is a nominal clause, not a verb.

Yes, يعتقد already tells you the subject is he because the verb form is 3rd person masculine singular.

So Arabic could simply say:

• يعتقد أن الموعد اليوم

and that would still mean he thinks that the appointment is today.

Adding هو can do a few things:

• make the subject clearer
• add emphasis
• help the sentence flow in context

So هو is not strictly necessary here, but it is completely normal.

يعتقد is an imperfect (non-past) verb, 3rd person masculine singular.

That means it can mean:

• he thinks
• he believes
• sometimes he is thinking / believing, depending on context

Its dictionary form is اعتقد.

So:

• هو يعتقد = he thinks / believes

الموعد has الـ, so it means the appointment / the date / the scheduled time, not just an appointment.

In Arabic, definiteness is shown very clearly:

• موعد = an appointment / a date
• الموعد = the appointment / the date

In this sentence, the speaker is referring to a specific appointment already known in context.

الموعد can mean several related things depending on context, such as:

• appointment
• date
• scheduled time
• sometimes meeting time

In this sentence, the exact English choice depends on the translation you were given. But grammatically, it is just the noun being talked about in the clause after أنَّ.

Because اليوم can function directly as an adverb of time, meaning today.

So:

• الموعد اليوم = the appointment is today

Arabic often uses time words directly like this, without needing something like on.

Compare the English pattern:

• The meeting is today
• not necessarily The meeting is on today

So Arabic is behaving quite naturally here.

It can do both, depending on context.

• اليوم can mean the day
• but very often it means today

In your sentence, it is functioning as a time expression, so it means today.

This is very common in Arabic: a word that historically contains الـ may still function like a standard time adverb in everyday meaning.

The order here is:

• أنَّ
• الموعد
• اليوم

This corresponds to:

• that
• the appointment
• today

In smoother English: that the appointment is today

The clause after أنَّ is basically a nominal sentence:

• الموعد اليوم

with الموعد as the topic/subject and اليوم as the predicate/time expression.

This is a very normal and straightforward order.

Yes, that is also possible.

Both of these can occur:

• يعتقد أن...
• يعتقد بأن...

The version without بـ is perfectly standard and very common. The version with بأن is also used and can sound slightly more formal or stylistically different depending on context.

For a learner, the important point is that أنَّ after يعتقد is completely correct.

Yes. In fully vocalized Arabic, أنَّ affects the following noun.

So the fully vocalized form would be:

• هُوَ يَعْتَقِدُ أَنَّ الْمَوْعِدَ الْيَوْمَ

Notice الموعدَ with fatḥa in full vocalization.

But in normal unvowelled writing, you usually just see:

• أن الموعد اليوم

So the grammatical change is there, even if the short vowel is not written.

In a sentence like this, اليوم functions as an adverbial expression of time, and adverbials of this type are often in the accusative.

So in full vocalization:

• الْمَوْعِدَ الْيَوْمَ

Both words end up with an a sound, but for different grammatical reasons:

• الموعدَ because it is the noun of أنَّ
• اليومَ because it is a time adverb / adverbial predicate

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

Yes, it is grammatical and natural.

It is a simple, standard MSA sentence meaning:

• He thinks that the appointment is today

A native speaker might also say similar variants depending on style and context, but the structure itself is very normal:

• pronoun + verb
• أنَّ
• nominal clause

Usually, with a verb like يعتقد, Arabic normally keeps أنَّ before a full clause like this.

So:

• هو يعتقد أن الموعد اليوم = normal

Leaving it out would generally not be standard in this structure.

In other words, أنَّ is doing important grammatical work by linking he thinks to the following statement.

It can be either, depending on context.

اعتقد / يعتقد often has the sense of:

• believe
• think
• consider

Compared with some other Arabic verbs, it can sound a little stronger or more formal than casual think, but in many contexts thinks that... is a perfectly good translation.

So in this sentence, English may translate it as either:

• He thinks that the appointment is today
• He believes that the appointment is today

Sometimes the Arabic imperfect can overlap with English is doing, but with a verb like يعتقد, the most natural translation is usually simple present:

• he thinks
• he believes

English he is thinking usually suggests an action happening right now in a more temporary way, which is not the main idea of يعتقد in most cases.

So for learners, the safest default here is he thinks / believes.

It would be:

• يعتقد أن الموعد اليوم

This still means he thinks that the appointment is today.

Arabic often omits subject pronouns because the verb already shows the person, number, and gender. Adding هو just makes the subject more explicit.