الحادث المروري صغير اليوم.

In Arabic, the verb to be is usually not expressed in the present tense when you are making a simple statement like X is Y.

So:

الحادث المروري صغير اليوم
literally works like: The traffic accident small today

This is a very common Arabic sentence pattern, called a nominal sentence.

If you wanted was or will be, then Arabic would use a verb, for example:

• كان الحادث المروري صغيرًا = The traffic accident was small
• سيكون الحادث المروري صغيرًا = The traffic accident will be small

Because they are doing different jobs in the sentence.

• الحادث المروري = the subject/topic: the traffic accident
• صغير = the predicate/comment: small

In Arabic nominal sentences, the subject is often definite, while the predicate is often indefinite.

So:

• الحادث = the accident
• المروري = traffic-related
• صغير = small

If you made صغير definite too (الصغير), it would usually change the structure and meaning, and it would no longer be the normal way to say is small.

المروري is an adjective meaning traffic-related or road-traffic.

It comes from مرور, which is connected with traffic/passing/movement on roads.

So:

• حادث = accident / incident
• مروري = traffic-related

Together:

• حادث مروري = a traffic accident
• الحادث المروري = the traffic accident

This is a very common Arabic way to build noun phrases: a noun followed by an adjective.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• حادث مروري = traffic accident
  literally: accident traffic-related

This is the standard Arabic order:

• noun + adjective

More examples:

• بيت كبير = a big house
• البيت الكبير = the big house
• سيارة سريعة = a fast car

So الحادث المروري follows the normal Arabic pattern.

Because حادث is a masculine singular noun, and the adjective must agree with it.

Arabic adjectives agree with the noun in:

• gender
• number
• definiteness
• and, in fully vowelled Arabic, case

So:

• حادث = masculine singular
• therefore صغير = masculine singular

If the noun were feminine, you would use صغيرة. For example:

• السيارة صغيرة = The car is small

Because المروري is an adjective describing الحادث, and Arabic adjectives must match the noun in definiteness.

So:

• حادث مروري = a traffic accident
• الحادث المروري = the traffic accident

You cannot normally say:

• الحادث مروري

if you mean the traffic accident, because the adjective should match the noun.

This matching is a very important rule in Arabic.

In fully vowelled Modern Standard Arabic, it would be:

الحادثُ المروريُّ صغيرٌ اليومَ

Here is why:

• الحادثُ = nominative, because it is the subject/topic (mubtadaʾ)
• المروريُّ = nominative too, because it is an adjective describing الحادث
• صغيرٌ = nominative, because it is the predicate (khabar)
• اليومَ = accusative as an adverb of time

In normal everyday writing, these short vowels are usually omitted, so you often just see:

الحادث المروري صغير اليوم

اليوم means today, and here it functions as an adverb of time.

It tells you when the statement applies:

• الحادث المروري صغير اليوم = The traffic accident is small today

In fully vowelled Arabic, it is often written اليومَ when used adverbially.

Arabic often places time expressions quite flexibly, so اليوم can appear in different positions depending on style and emphasis.

Yes. Arabic word order is fairly flexible, especially for adverbs like اليوم.

All of these can work, with slightly different emphasis:

• الحادث المروري صغير اليوم
• اليوم الحادث المروري صغير
• الحادث المروري اليوم صغير

The version you were given is perfectly normal. Putting اليوم at the end is a simple, neutral choice.

A careful pronunciation with full endings would be:

al-ḥādithu al-murūriyyu ṣaghīrun al-yawma

A few notes:

• ح in حادث is a strong ḥ sound, not like English h
• الـ is pronounced fully here because the next letters are moon letters, not sun letters
• مروري has a long ū sound: mu-rū-riyy
• ص in صغير is an emphatic s

Without case endings, learners often say something closer to:

al-ḥādith al-murūrī ṣaghīr al-yawm

It is understandable, but depending on context, صغير may sound a little literal.

For accidents, Arabic often uses words like:

• بسيط = minor / simple
• طفيف = slight / minor

So a more idiomatic sentence might be:

• الحادث المروري بسيط اليوم = The traffic accident is minor today

Still, صغير is grammatically fine, and for a learner it is useful because it clearly shows the sentence structure.