صندوق المصاهر قديم جدا.

In present-tense Arabic, a sentence like this usually does not use a separate word for is/are. So:

صندوق المصاهر قديم جدا

literally looks like box of the ... old very, but it naturally means The ... box is very old.

This is a normal nominal sentence in Arabic. If you wanted was, Arabic would usually use a verb such as كان.

It is an iḍāfa construction, often called a genitive construction or possessive construction.

It works like this:

• صندوق = the first noun
• المصاهر = the second noun

Together they make a phrase meaning something like the box of the ... or the ... box, depending on the meaning already given to you.

In an iḍāfa:

• the first noun is called the muḍāf
• the second noun is called the muḍāf ilayh

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

So Arabic says:

• صندوق المصاهر

not:

• الصندوق المصاهر for this meaning

The definiteness of the whole phrase is determined by the second noun. Since المصاهر has الـ, the whole phrase is definite.

So صندوق المصاهر can mean the box of the ... even though صندوق itself does not have الـ.

Because the first noun in an iḍāfa cannot take tanwīn.

So in full grammatical form, it would be:

• صندوقُ المصاهرِ

not:

• صندوقٌ المصاهرِ

This is one of the standard signs that a noun is functioning as the first part of an iḍāfa.

Because قديم agrees with the noun it describes, and the head noun here is صندوق, which is masculine singular.

So Arabic uses:

• قديم = masculine singular

not a feminine or plural form.

A key point: even though المصاهر may look like the more noticeable noun at the end of the phrase, the adjective agrees with صندوق, because that is the main noun of the iḍāfa.

Because قديم here is the predicate of the sentence, not an adjective inside the noun phrase.

So:

• صندوق المصاهر قديم = The ... box is old

But:

• صندوق المصاهر القديم = the old ... box

That second version is just a noun phrase, not a complete sentence.

In Arabic, the predicate in a nominal sentence is often indefinite, which is why قديم appears without الـ.

Because جداً normally comes after the adjective or adverb it strengthens.

So Arabic says:

• قديم جدًا = very old

This is the usual word order in Modern Standard Arabic.

Also, in fully vowelled writing it is normally written:

• جدًّا

In everyday unvowelled writing, you will often see:

• جدا

A fully vowelled version would be:

صندوقُ المصاهرِ قديمٌ جدًّا

Here is what the endings show:

• صندوقُ = nominative, because it is the subject/topic of the nominal sentence
• المصاهرِ = genitive, because it is the second term of the iḍāfa
• قديمٌ = nominative, because it is the predicate
• جدًّا = accusative adverbial form

In normal Arabic writing, these short vowels are usually omitted.

It is pronounced clearly as al-, because م is a moon letter, not a sun letter.

So the beginning is pronounced like:

• al-ma...

not with assimilation.

If the learner is paying attention to pronunciation, another useful point is that ص is an emphatic consonant, so it is not exactly the same as a plain English s.

Usually, no. In careful grammatical reading, you can pronounce the full endings:

• صندوقُ المصاهرِ قديمٌ جدًّا

But in normal pausal pronunciation, many final short vowels are dropped. So it may sound more like:

• صندوق المصاهر قديم جدا

This is why learners often see the sentence written without endings and hear a reduced pronunciation in actual reading.