الصيانة مهمة.

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

So الصيانة مهمة literally looks like maintenance important, but it means Maintenance is important.

If you wanted a past-tense version, Arabic would normally use a form of كان:

• كانت الصيانة مهمة = Maintenance was important

This kind of sentence is called a nominal sentence in Arabic.

ال is the Arabic definite article, equivalent to the in English.

So:

• صيانة = maintenance / upkeep
• الصيانة = the maintenance

In many Arabic sentences like this, English may translate it more naturally without the, depending on context. So الصيانة مهمة can still be translated as Maintenance is important rather than only The maintenance is important.

Because الصيانة is a feminine noun, the adjective describing it must also be feminine.

So:

• masculine adjective: مهم = important
• feminine adjective: مهمة = important

Since الصيانة is feminine, Arabic says:

• الصيانة مهمة

This is called agreement: the adjective matches the noun in gender.

A very common clue is the ending ـة (called taa marbuuTa).

The word صيانة ends in ـة, and many nouns with this ending are feminine.

That does not mean every feminine noun must end this way, or that every word ending this way is always simple to classify, but it is one of the strongest patterns learners use.

In this sentence, مهمة is not being used as the important one. It is the predicate of the sentence: it tells you something about الصيانة.

So Arabic commonly has this pattern:

• المبتدأ (topic/subject): الصيانة
• الخبر (predicate/comment): مهمة

That is why you get:

• الصيانة مهمة = Maintenance is important

If you said الصيانة المهمة, that would usually mean the important maintenance, which is a noun phrase, not a full sentence.

In fully vowelled, formal Arabic grammar, it is:

• الصيانةُ مهمةٌ

The endings are:

• الصيانةُ with -u because it is the subject of the nominal sentence
• مهمةٌ with -un because it is the predicate and indefinite

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

• الصيانة مهمة

The word is pronounced approximately aṣ-ṣiyaanah.

The ل of ال is not pronounced here because ص is a sun letter. With sun letters, the l sound assimilates into the next consonant.

So:

• written: الصيانة
• pronounced: aṣ-ṣiyaanah

You will hear a doubled ṣ sound.

By contrast, with a moon letter, the l is pronounced normally.

It is a noun, and more specifically it is often understood as a verbal noun meaning something like maintenance, upkeep, or servicing.

So the sentence is not using a verb like to maintain. Instead, it uses the noun maintenance:

• الصيانة مهمة = Maintenance is important

Not if you want the same sentence meaning.

• الصيانة مهمة = Maintenance is important
  This is a complete sentence.

But:

• مهمة الصيانة

usually means something like the task of maintenance or maintenance task, depending on context. It becomes more like a noun phrase, not the same statement.

So for Maintenance is important, keep:

• الصيانة مهمة

Yes. Both مهم and هام can mean important.

So both of these are possible:

• الصيانة مهمة
• الصيانة هامة

For many learners, مهم / مهمة is the more common everyday word to learn first.

Because Arabic adjectives behave like nouns in many ways and show gender.

The ending ـة here marks the adjective as feminine singular:

• مهم = masculine singular
• مهمة = feminine singular

Since الصيانة is feminine singular, the adjective matches it:

• الصيانة مهمة

It is a nominal sentence because it begins with a noun, not a verb.

Its two parts are:

• الصيانة = the topic/subject
• مهمة = the comment/predicate

This is one of the most common sentence patterns in Arabic, especially for statements like:

• الجو جميل = The weather is nice
• الكتاب مفيد = The book is useful
• الصيانة مهمة = Maintenance is important

A natural pronunciation is approximately:

aṣ-ṣiyaanatu muhimmah

If you are not pronouncing case endings, a simpler classroom pronunciation is:

aṣ-ṣiyaanah muhimmah

A few sound notes:

• ṣ is an emphatic s
• yaa in صيانة is a long vowel sound
• the final ة in pause is pronounced like -ah
• مهمة has doubled m in careful pronunciation: mu-him-mah