إعلان المحطة مهم.

Because this is a present-tense nominal sentence in Arabic. In Modern Standard Arabic, sentences like X is Y often have no written or spoken word for is in the present tense.

So:

إعلان المحطة مهم.
literally = The station’s announcement important.
natural English = The station’s announcement is important.

If you wanted was or will be, Arabic would usually use a verb such as كان or سيكون.

It is an iḍāfa construction, often called a genitive construct or noun + noun possession structure.

So:

• إعلان = announcement
• المحطة = the station

Together, إعلان المحطة means:

• the station’s announcement
• or more literally, announcement of the station

This is one of the most common ways Arabic expresses possession or close relationships between nouns.

Because in an iḍāfa construction, the first noun normally does not take ال.

So Arabic does not say:

• الإعلان المحطة ❌

Instead it says:

• إعلان المحطة ✅

Even without الـ on إعلان, the whole phrase can still be definite because the second noun, المحطة, is definite.

So إعلان المحطة is definite overall and means the station’s announcement, not just a station’s announcement.

Because المحطة is the second noun in the iḍāfa, and it is the noun that makes the whole phrase definite.

Compare:

• إعلان محطة = a station’s announcement / announcement of a station
• إعلان المحطة = the station’s announcement

So the الـ on المحطة is what gives the phrase the meaning the station’s ...

In an iḍāfa construction, the second noun is always genitive.

If full case endings are written, the sentence is:

إِعْلانُ المَحَطَّةِ مُهِمٌّ

Here:

• إعلانُ = nominative, because it is the subject of the sentence
• المحطةِ = genitive, because it is the second term of the iḍāfa
• مهمٌّ = nominative, because it is the predicate

In normal everyday Arabic writing, these short vowel endings are usually omitted, so you just see:

إعلان المحطة مهم

Because it refers to إعلان, and إعلان is a masculine singular noun.

So the predicate is also masculine singular:

• إعلان ... مهم = The announcement ... is important

If the subject were feminine, the predicate would usually be feminine too. For example:

• الرسالة مهمة = The message is important

So مهم matches إعلان, not المحطة.

It is functioning as the predicate of the sentence, called خبر in Arabic grammar.

In other words:

• إعلان المحطة = the subject
• مهم = what is being said about the subject

The word مهم is the same form you would also use as an adjective in other contexts, but here its job is predicative, meaning is important rather than directly modifying a noun inside the noun phrase.

Because the normal order in this kind of Arabic nominal sentence is:

subject + predicate

So:

• إعلان المحطة = subject
• مهم = predicate

That gives:

إعلان المحطة مهم

This is the standard and natural order for a simple statement like this.

A fully vocalized version is:

إِعْلانُ المَحَطَّةِ مُهِمٌّ

Word by word:

• إِعْلانُ = iʿlānu
• المَحَطَّةِ = al-maḥaṭṭati
• مُهِمٌّ = muhimmun

In ordinary texts, these case vowels are usually not written, so learners often see the simpler form:

إعلان المحطة مهم

Usually, when people pause at the end of the word, ة is pronounced like -a / -ah, not a strong t sound.

So المحطة is commonly pronounced roughly:

al-maḥaṭṭa

But in full grammatical reading with case endings, the t is heard before the ending:

al-maḥaṭṭati

So both ideas are useful:

• citation / pause form: al-maḥaṭṭa
• full grammatical form: al-maḥaṭṭati

Not always. إعلان can also mean:

• announcement
• advertisement
• ad

The exact meaning depends on context.

In إعلان المحطة, a learner will often understand it as the station’s announcement, which fits very naturally. But in another context, إعلان could refer to an advertisement or commercial.

That is a key feature of iḍāfa.

In Arabic, the definiteness of the whole phrase is determined by the second noun. Since المحطة is definite, the whole phrase إعلان المحطة is also definite.

So:

• إعلان محطة = indefinite
• إعلان المحطة = definite

This is why Arabic can express the station’s announcement without putting الـ on the first noun.