अगर हम अभी जाएँगे, तो हम जल्दी शहर पहुँचेंगे।

अगर ... तो ... is the Hindi pattern for if ... then ....

• अगर introduces the condition.
• तो introduces the result.

So the structure is literally if ..., then ....
तो is very common, but not always absolutely required. You may hear sentences without it, especially in speech, but using it makes the relationship between the two clauses clearer.

This is a very common learner question, and it matters here.

In the sentence as written, जाएँगे is a future tense form: we will go. But in standard Hindi, after अगर in a real future condition, many speakers would more naturally say:

अगर हम अभी जाएँ, तो हम जल्दी शहर पहुँचेंगे।

Here जाएँ is a subjunctive-style form, which is very common in if clauses.

So:

• Given sentence: अगर हम अभी जाएँगे...
• More natural for most learners to copy: अगर हम अभी जाएँ...

Using future in both clauses can occur in some speech, but if you want safe, standard Hindi, अगर + जाएँ + future result is the better pattern.

Yes, you can often leave the second हम out.

Hindi frequently repeats the subject in both clauses for clarity:

अगर हम अभी जाएँ, तो हम जल्दी शहर पहुँचेंगे।

But it is also natural to say:

अगर हम अभी जाएँ, तो जल्दी शहर पहुँचेंगे।

And if the context is very clear, Hindi often drops pronouns even more than English does. Repeating हम is not wrong; it just sounds a bit more explicit.

The ending -ेंगे is a future ending used here with हम.

In this sentence it shows:

• future
• plural
• usually masculine or mixed-gender group

So:

• हम जाएँगे = we will go
• हम पहुँचेंगे = we will reach

If the group were all female, you would normally say:

• हम जाएँगी
• हम पहुँचेंगी

So the verb form changes with number and gender.

अभी means now or right now.

जल्दी can mean several things depending on context:

• quickly
• soon
• early
• sooner

In this sentence, जल्दी does not mainly mean moving fast. It means something more like sooner / earlier, because leaving now will make the arrival happen earlier.

So the idea is not we will travel fast, but we will arrive sooner if we leave now.

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

So शहर can mean:

• city
• a city
• the city

The exact meaning comes from context. If you need to be more specific, Hindi uses other words, such as:

• यह शहर = this city
• वह शहर = that city
• उस शहर = that city / that particular city

With पहुँचना (to reach / arrive), Hindi often uses the destination directly, without a postposition.

So these are normal:

• घर पहुँचना = to reach home
• स्टेशन पहुँचना = to reach the station
• शहर पहुँचना = to reach the city

English often says both reach the city and arrive in the city, but Hindi commonly uses the direct destination pattern here.
So शहर पहुँचेंगे is very natural.

जाना is understandable, but if the real idea is leave now, many speakers would choose a verb like:

• चलना = to get going / set off
• निकलना = to leave / depart

So a very natural version would be:

अगर हम अभी निकलें, तो हम जल्दी शहर पहुँचेंगे।
or
अगर हम अभी चलें, तो हम जल्दी शहर पहुँचेंगे।

These versions emphasize the start of the trip more clearly. जाना is still possible, but निकलना or चलना often sounds more idiomatic in this situation.