يحترق الصندوق قرب إنذار الحريق.

يحترق is the imperfect form of احترق, which means to burn, to catch fire, or to be on fire.

In this sentence, it means that the box itself is burning.

A useful point:

• يحترق can mean burns or is burning
• Arabic often lets context tell you whether the action is general or happening right now

So here it is naturally understood as something like is burning.

Because يحترق and يحرق are not the same verb.

• يحترق = burns / is burning itself
  
  • يحترق الصندوق = the box is burning
• يحرق = burns something / sets something on fire
  
  • يحرق الرجل الصندوق = the man burns the box

So يحترق is the right choice when the subject is the thing that is on fire.

Because verb-first word order is very common in Modern Standard Arabic.

So:

• يحترق الصندوق = a normal, natural Arabic sentence
• الصندوق يحترق = also correct, but it puts more focus on the box

English usually prefers subject + verb, but Arabic often prefers verb + subject, especially in neutral statements.

Arabic does not need a separate present-tense is here.

English says:

• The box is burning

Arabic simply uses the imperfect verb:

• يحترق الصندوق

So the idea of is burning is built into the verb and the context. Arabic does not need an extra word like English is in this kind of sentence.

Because the subject الصندوق is singular and grammatically masculine.

So the verb matches it:

• يحترق = he/it burns masculine singular

If the subject were feminine singular, the verb would change:

• تحترق السيارة = the car is burning

Here قرب means near or close to.

So:

• قرب إنذار الحريق = near the fire alarm

In grammar, قرب is often treated as a noun/adverb of place, but for a learner it is fine to think of it simply as near.

A very common alternative in Arabic is:

• بالقرب من إنذار الحريق = near the fire alarm

Because this is an iḍāfa construction, often called a construct phrase.

Structure:

• إنذار = alarm
• الحريق = the fire

Together:

• إنذار الحريق = literally alarm of the fire
• natural English: fire alarm

This is how Arabic often expresses noun + noun combinations that English expresses with compounds.

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

So:

• إنذار الحريق is correct
• الإنذار الحريق is not correct for this meaning

The whole phrase becomes definite because the second noun is definite:

• الحريق = the fire
• therefore إنذار الحريق = the fire alarm

Also, the first noun in iḍāfa does not take tanwīn either.

Fully vocalized, the sentence is:

يَحْتَرِقُ الصُّنْدُوقُ قُرْبَ إِنْذَارِ الحَرِيقِ

Here is why:

• يَحْتَرِقُ: indicative imperfect, so it ends in -u
• الصُّنْدُوقُ: the subject, so it is nominative
• قُرْبَ: used here as an adverbial expression of place, so it is in the accusative
• إِنْذَارِ: genitive in relation to قرب
• الحَرِيقِ: genitive because it is the second term of the iḍāfa

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

Although it is written الصندوق, it is pronounced with assimilation because ص is a sun letter.

So it is pronounced approximately:

• aṣ-ṣundūq

not:

• al-ṣundūq

By contrast, in الحريق, the ل of الـ is pronounced, because ح is not a sun letter.

So:

• الحريق ≈ al-ḥarīq

Yes. The Arabic imperfect can cover several meanings depending on context, including:

• is burning right now
• burns in a general sense
• sometimes a habitual or repeated action

In this sentence, context will usually make is burning the most natural reading, but the verb form itself is flexible in a way that English often is not.