आज बस बहुत धीरे जा रही है, इसलिए हम देर से पहुँचेंगे।

Because बस is grammatically feminine in Hindi.

In the progressive form:

• जा रहा है = masculine singular
• जा रही है = feminine singular
• जा रहे हैं = masculine plural / mixed plural

Since बस is feminine singular, the correct form is जा रही है.

It is the present progressive form of जाना (to go).

It breaks down like this:

• जा = the verb stem from जाना
• रही = marks an ongoing action for a feminine subject
• है = is

So जा रही है literally means is going or is in the process of going.

In this sentence, it means the bus is currently moving slowly.

Because धीरे is being used like an adverb, modifying the verb जा रही है.

• धीरे = slowly
• धीमी = slow, feminine adjective

So:

• धीरे जा रही है = is going slowly
• धीमी बस = a slow bus

English uses slow and slowly differently, and Hindi does something similar here.

बहुत means very or a lot.

Here it modifies धीरे, so:

• धीरे = slowly
• बहुत धीरे = very slowly

It does not change form here. Hindi adverbs like this are usually simple and do not inflect.

Putting आज at the beginning is very natural in Hindi when you want to set the time first.

So the sentence starts by establishing the context:

• आज = today
• then what is happening today

Hindi word order is flexible, but the beginning of the sentence is a common place for time words such as:

• आज = today
• कल = tomorrow / yesterday
• अब = now

You could move it, but the given order sounds very natural.

इसलिए means therefore, so, or for that reason.

It links:

1. the cause: बस बहुत धीरे जा रही है
2. the result: हम देर से पहुँचेंगे

So the structure is:

X, इसलिए Y
= X, so Y

It is a very common and useful connector in Hindi.

देर से is the normal Hindi way to say late in this kind of sentence.

So:

• देर से पहुँचना = to arrive late

Here से is part of the expression. It does not translate neatly word-for-word as from. Instead, the whole phrase देर से works adverbially.

Compare:

• वह जल्दी पहुँचा। = He arrived early.
• वह देर से पहुँचा। = He arrived late.

So for learners, it is best to remember देर से as a set phrase meaning late.

Because the subject is हम (we).

Hindi verbs usually agree with the subject in person and number, and often also gender. So with हम, the verb is plural.

• मैं पहुँचूँगा / पहुँचूँगी = I will arrive
• हम पहुँचेंगे = we will arrive

In many contexts, पहुँचेंगे is used for a masculine or mixed group. If the group were entirely female, you may also hear पहुँचेंगी.

Because Hindi does not use articles like a, an, and the the way English does.

So बस can mean:

• a bus
• the bus

The exact meaning depends on context.

That is very normal in Hindi, and learners need to get used to understanding definiteness from the situation rather than from an article.

Yes, absolutely. That would sound very natural too.

There is a slight nuance:

• जा रही है focuses on going
• चल रही है focuses on moving / running

With vehicles, both are common in everyday Hindi. So:

• बस बहुत धीरे जा रही है
• बस बहुत धीरे चल रही है

Both can mean that the bus is moving very slowly.

Hindi usually prefers Subject–Object–Verb, and the verb often comes at the end of the clause.

This sentence is structured like this:

• आज = time expression
• बस = subject
• बहुत धीरे = adverb phrase
• जा रही है = verb

Then:

• इसलिए = connector
• हम = subject
• देर से = adverb phrase
• पहुँचेंगे = verb

So even if some parts move around, the big pattern is that the main verb tends to come last in each clause. That is one of the most important word-order habits to learn in Hindi.