الطبيب ليس في المستشفى اليوم.

In Modern Standard Arabic, a present-tense sentence like The doctor is in the hospital usually does not need a word for is.

So:

• الطبيب في المستشفى = The doctor is in the hospital

Arabic often leaves the present-tense to be unspoken in sentences like this.

When you want to make that kind of sentence negative, Arabic commonly uses ليس:

• الطبيب ليس في المستشفى = The doctor is not in the hospital

So ليس is doing the job of is not here.

ليس is the normal way to negate a nominal sentence like this one.

This sentence is built around a noun phrase:

• الطبيب = the doctor
• في المستشفى = in the hospital

So to say is not, Arabic uses ليس.

A learner might want to say something like الطبيب لا في المستشفى, but that is not standard Arabic.

Very roughly:

• ليس negates sentences like X is Y / X is in Y
• لا is often used to negate verbs or in other special patterns

So here, ليس is the correct choice.

Yes. ليس agrees with the subject in person, gender, and number.

Here the subject is الطبيب, which is masculine singular, so Arabic uses ليس.

Compare:

• الطبيب ليس في المستشفى اليوم.
  The male doctor is not in the hospital today.

• الطبيبة ليست في المستشفى اليوم.
  The female doctor is not in the hospital today.

• الطبيبان ليسا في المستشفى اليوم.
  The two doctors are not in the hospital today.

• الأطباء ليسوا في المستشفى اليوم.
  The doctors are not in the hospital today.

So ليس is not a fixed form in every sentence.

Because this is a very normal noun-first sentence pattern in Arabic.

The sentence starts with the topic or subject:

• الطبيب = the doctor

Then it gives information about that subject:

• ليس في المستشفى اليوم = is not in the hospital today

So the structure is basically:

• الطبيب = subject/topic
• ليس في المستشفى اليوم = what is being said about the subject

This is a very common and natural word order in Arabic.

Because they are both definite.

• الطبيب = the doctor
• المستشفى = the hospital

Without الـ, they would usually be indefinite:

• طبيب = a doctor / doctor
• مستشفى = a hospital / hospital

So the sentence is talking about a specific doctor and a specific hospital, or at least treating them as known in context.

Because of the sun letter / moon letter rule.

In الطبيب, the first letter after الـ is ط, which is a sun letter. That means the ل sound of الـ is not pronounced separately. Instead, the next consonant is doubled.

So الطبيب is pronounced approximately:

• aṭ-ṭabīb

not:

• al-ṭabīb

But in المستشفى, the first letter after الـ is م, which is a moon letter. So the ل sound is pronounced normally:

• al-mustashfā

So:

• الطبيب → the l sound disappears in pronunciation
• المستشفى → the l sound stays

في is the preposition meaning in or sometimes at, depending on context.

So:

• في المستشفى = in the hospital or at the hospital

This is how Arabic expresses location here.

Also, after a preposition like في, the following noun is grammatically in the genitive case. In normal unvowelled writing, you usually do not see that clearly, and with المستشفى the ending is not visibly marked anyway.

The final ى is called alif maqṣūrah. It looks a bit like ي without dots, but here it represents a final long ā sound.

So المستشفى is pronounced:

• al-mustashfā

not:

• al-mustashfi
• al-mustashfaya

This is a very common thing in Arabic spelling, and learners often notice it quickly.

Because in Arabic, اليوم is the normal word for today, even though it literally contains الـ.

So:

• اليوم = today

This is just how Arabic commonly expresses it. You should learn اليوم as a standard time expression.

So in the sentence:

• الطبيب ليس في المستشفى اليوم.
  = The doctor is not in the hospital today.

the word اليوم is functioning as an adverb of time.

Yes, it can move, but the given order is very natural.

Arabic often places time expressions like اليوم at the end:

• الطبيب ليس في المستشفى اليوم.

That sounds neutral and straightforward.

But Arabic word order is flexible, so you could also move اليوم earlier for emphasis, for example:

• اليوم الطبيب ليس في المستشفى.

That can sound more like Today, the doctor is not in the hospital.

So the end position is common, but not the only possibility.

A fully vowelled version would be:

• الطَّبيبُ ليسَ في المُستَشْفَى اليَومَ

Yes, there are case endings here, even though they are usually not written in everyday Arabic.

A simple breakdown:

• الطبيبُ: nominative, because it is the subject of ليس
• ليسَ: the negative verb
• في المستشفى: prepositional phrase, in the hospital
• اليومَ: an adverbial expression of time, often in the accusative

One important point: with المستشفى, the case ending is not visibly shown in the usual spelling because of the final ى.

So even though case still matters grammatically, learners often will not see it clearly in normal text.