الصيانة ضرورية.

A careful MSA pronunciation is aṣ-ṣiyānatu ḍarūriyyatun.

In normal pause, which is how people usually say it aloud, it is more like:

aṣ-ṣiyāna ḍarūriyya

A few useful notes:

• In الصيانة, the ال is pronounced with assimilation because ص is a sun letter, so you hear aṣ-ṣ..., not al-ṣ...
• The final ة is usually heard as -a in pause.
• In fully vocalized formal Arabic, the endings are -u and -un, but these are often not pronounced in ordinary reading or speech.

It is a noun, not a verb.

More specifically, it is a verbal noun or abstract noun meaning the idea of maintenance. So the sentence is built from:

• الصيانة = the subject, a noun
• ضرورية = the predicate adjective, meaning necessary

This is why the whole sentence is a nominal sentence, not a verbal sentence.

In Arabic, the verb to be is usually not stated in the present tense.

So الصيانة ضرورية literally looks like maintenance necessary, but in natural English it means maintenance is necessary.

If you wanted a past or future meaning, Arabic would normally use a verb, for example:

• كانت الصيانة ضرورية = maintenance was necessary
• ستكون الصيانة ضرورية = maintenance will be necessary

Because الصيانة is grammatically feminine, and the adjective must agree with it.

The masculine form is:

• ضروري = necessary

The feminine form is:

• ضرورية = necessary

Since الصيانة is feminine singular, the predicate adjective also appears as feminine singular: ضرورية.

The final ة, called tāʾ marbūṭa, often marks a word as feminine.

In this sentence:

• الصيانة ends in ة and is feminine
• ضرورية also has a feminine ending, because it agrees with الصيانة

In pause, ة is usually pronounced like -a.
When grammar endings are fully pronounced, it can sound like -atun or -atu depending on the form.

Because here ضرورية is not being used as an attributive adjective inside a noun phrase. It is the predicate of the sentence.

So:

• الصيانة ضرورية = maintenance is necessary

But:

• الصيانة الضرورية = the necessary maintenance

That second one is a noun phrase, not the same sentence structure.

This is a very important difference in Arabic:

• definite noun + indefinite adjective/predicate often makes a full sentence
• definite noun + definite adjective makes a noun phrase

They look similar, but they mean different things grammatically.

• الصيانة ضرورية = a complete sentence: maintenance is necessary
• الصيانة الضرورية = a noun phrase: the necessary maintenance

Why?

In الصيانة ضرورية:

• الصيانة is the subject
• ضرورية is the predicate

In الصيانة الضرورية:

• الضرورية directly describes الصيانة
• both words are definite, so they form one unit: the necessary maintenance

In fully vocalized formal Arabic, the sentence is:

الصيانةُ ضروريةٌ

Both words are in the nominative case because:

• الصيانةُ is the subject of a nominal sentence
• ضروريةٌ is the predicate, and it is also nominative

In most everyday writing, those short vowels are omitted, so you usually just see:

الصيانة ضرورية

For a beginner, the normal and safest order is:

الصيانة ضرورية

That is the standard neutral order: subject + predicate.

A reversed order can sometimes happen for style, emphasis, or special contexts, but it is not the basic pattern you should learn first. So if you want to say this plainly in MSA, keep it as:

الصيانة ضرورية

Yes, but the meaning changes slightly.

• الصيانة ضرورية = maintenance is necessary / essential
• الصيانة مهمة = maintenance is important

So ضرورية is usually stronger than مهمة.

If you want to say that maintenance is not just useful but truly required, ضرورية is the better choice.