مطفأة الحريق منتهية الصلاحية.

Because Arabic usually leaves out the present-tense verb to be. So مطفأة الحريق منتهية الصلاحية is a normal present-tense nominal sentence.

A very literal breakdown is:

• مطفأة الحريق = the fire extinguisher
• منتهية الصلاحية = expired / past its valid period

So the full meaning is simply The fire extinguisher is expired or more naturally The fire extinguisher has expired.

It is an iḍāfa construction, often called a construct phrase.

That means two nouns are put together:

• مطفأة = extinguisher
• الحريق = the fire

Literally, it is extinguisher of the fire, but in natural English we say fire extinguisher.

This noun+noun structure is very common in Arabic.

Because in an iḍāfa construction, the first word normally does not take الـ.

So Arabic says:

• مطفأة الحريق

not:

• المطفأة الحريق

The definiteness comes from the second word. Since الحريق has الـ, the whole phrase becomes definite: the fire extinguisher.

Because it refers to مطفأة, and مطفأة is grammatically feminine.

You can often recognize this from the final ة. In Arabic, adjectives and similar describing words usually agree with the noun in gender and number.

So:

• feminine: منتهية الصلاحية
• masculine: منتهي الصلاحية

For example, with a masculine noun like الدواء you would say الدواء منتهي الصلاحية.

It is a common fixed expression meaning expired or past its expiration date.

A rough word-by-word sense is:

• منتهية = ended / finished
• الصلاحية = validity / usability / fitness for use

So the whole phrase means something like whose validity has ended.

In natural English, you usually should not translate it word for word. Just learn منتهي/منتهية الصلاحية as the normal Arabic expression for expired.

Yes, it behaves like one.

The first part is منتهية, and the second part is الصلاحية. Together they form a fixed phrase meaning expired.

This is useful to notice because it explains two things:

• منتهية does not take الـ
• الصلاحية comes after it as the second part of the phrase

So the sentence actually contains two linked constructions:

• مطفأة الحريق
• منتهية الصلاحية

When a word ending in ة is said by itself, it is usually pronounced like -a or -ah in pause.

But when that word is the first part of an iḍāfa, the ة is pronounced as t.

So in careful reading:

• مطفأة الحريق sounds like miṭfaʾat al-ḥarīq
• منتهية الصلاحية sounds like muntahiyat aṣ-ṣalāḥiyya

This is a very common feature of ة in Arabic.

It is a nominal sentence with:

• مطفأة الحريق as the subject
• منتهية الصلاحية as the predicate

In traditional Arabic grammar, these are called:

• مبتدأ for the subject
• خبر for the predicate

So the structure is basically:

• [subject] + [description/predicate]

That is one of the most common sentence patterns in Modern Standard Arabic.

Yes. A more verbal way to express the same idea is:

• انتهت صلاحية مطفأة الحريق

Literally: The validity of the fire extinguisher ended.

Both are natural, but the version you were given, مطفأة الحريق منتهية الصلاحية, sounds especially natural in labels, notices, inspection contexts, or short written statements.