दमकल जल्दी आती है।

Because दमकल is treated as a feminine singular noun here.

In Hindi, the main part of the verb in this kind of sentence agrees with the subject:

• masculine singular: आता है
• feminine singular: आती है
• plural: आते हैं or आती हैं, depending on gender

So with दमकल, Hindi uses आती है.

This is how the habitual present is built in Hindi:

• आती = the agreeing participle form
• है = the present-tense form of होना (to be)

Together, आती है means comes or does come.

In standard Hindi, leaving out है would usually sound incomplete in a sentence like this.

जल्दी is an adverb. Here it can mean:

• quickly
• soon
• early

With आना (to come), the exact meaning depends on context. So दमकल जल्दी आती है could mean either the fire engine comes quickly or the fire engine arrives soon.

No. जल्दी is an adverb, so it does not change for gender or number.

That is different from words like आता / आती / आते, which do change to agree with the subject.

It is the habitual present (often called the simple present in beginner materials).

It usually describes:

• a general fact
• a usual action
• a repeated pattern

So this sentence sounds like a general statement such as The fire engine comes quickly or The fire brigade usually arrives quickly.

For something happening right now, Hindi usually uses the progressive:

• दमकल जल्दी आ रही है।

Compare:

• दमकल जल्दी आती है। = it comes quickly / it usually arrives quickly
• दमकल जल्दी आ रही है। = it is coming quickly right now

The dictionary form is आना, which means to come.

From आना, you get forms like:

• आता है — comes (masculine singular)
• आती है — comes (feminine singular)
• आते हैं — come (plural or masculine plural)

Hindi does not have articles like a, an, and the.

So दमकल can mean:

• a fire engine
• the fire engine

The context tells you which one is meant.

The neutral order here is:

subject + adverb + verb

So:

• दमकल जल्दी आती है।

This is very natural Hindi. Hindi word order is somewhat flexible, so you may hear other orders for emphasis, but the verb usually stays near the end.

The verb would change to match the new subject.

For example:

• ट्रक जल्दी आता है। — The truck comes quickly.
  masculine singular

• गाड़ी जल्दी आती है। — The car comes quickly.
  feminine singular

• गाड़ियाँ जल्दी आती हैं। — The cars come quickly.
  feminine plural

So in your sentence, आती है tells you that the subject is being treated as feminine singular.