हालाँकि नियत तारीख कल है, फिर भी मैं उसे आज लौटाऊँगी।

Yes, they work together.

• हालाँकि introduces the concessive idea: although
• फिर भी reinforces the contrast in the main clause: even so / still / nevertheless

So the pattern is:

• हालाँकि X, फिर भी Y
• Although X, still Y

In English, using both can sometimes sound a bit heavy, but in Hindi this pairing is very common and natural.

You could omit फिर भी and say:

• हालाँकि नियत तारीख कल है, मैं उसे आज लौटाऊँगी।

That is still correct, but फिर भी makes the contrast clearer and more emphatic.

नियत तारीख means fixed date, set date, or in this context, due date.

Breakdown:

• नियत = fixed, निर्धारित, decided in advance
• तारीख = date

So together, नियत तारीख is the date that has already been set.

In everyday English for this sentence, due date is the most natural translation.

Hindi word order is flexible, but the usual neutral pattern puts the topic first and the verb at the end:

• नियत तारीख कल है
• literally: The due date tomorrow is

This is very normal Hindi structure.

You could also say:

• कल नियत तारीख है

but that shifts the emphasis a bit toward कल. It can sound like Tomorrow is the due date.

So both are possible, but:

• नियत तारीख कल है = neutral, natural here
• कल नियत तारीख है = more emphasis on tomorrow

कल can mean either yesterday or tomorrow. Hindi uses the same word for both.

You tell from context.

Here, it means tomorrow because:

• the sentence talks about a due date
• the speaker says I will return it today
• the verb लौटाऊँगी is future

So the meaning is clearly:

• Although the due date is tomorrow, I’ll return it today anyway.

If the context were about the past, कल could mean yesterday instead.

उसे is a pronoun form that can mean:

• him
• her
• it
• to him / to her / to it

Here it means it.

It comes from वह and is an oblique/object form. In real usage, उसे is often used for a previously mentioned thing or person that is not near the speaker.

The contrast is usually:

• इसे = this / this one / this thing here
• उसे = that / that one / that thing there / that previously mentioned thing

So if the item has already been mentioned and is not being treated as something physically near the speaker, उसे is very natural.

This is a very important distinction.

• लौटना = to return, come back, go back
• लौटाना = to return something, give something back

So:

• मैं लौटूँगी = I will return / come back
• मैं उसे लौटाऊँगी = I will return it

In this sentence, the speaker is returning an object, not returning herself somewhere. That is why the transitive verb लौटाना is used, and the future feminine form is लौटाऊँगी.

The ending -ऊँगी shows:

• future tense
• first person singular: I
• feminine speaker

So लौटाऊँगी means I will return it, said by a female speaker.

If the speaker were male, it would be:

• लौटाऊँगा

This gender agreement is very common in Hindi verbs.

It could be dropped if the context already makes the subject clear.

Hindi often leaves out subject pronouns when they are understood.

So both are possible:

• हालाँकि नियत तारीख कल है, फिर भी मैं उसे आज लौटाऊँगी।
• हालाँकि नियत तारीख कल है, फिर भी उसे आज लौटाऊँगी।

The version with मैं is a bit clearer and can also add slight emphasis: I will return it today.

In Hindi, time words such as आज, कल, अभी, शाम को often come before the main verb.

So:

• मैं उसे आज लौटाऊँगी
• literally: I it today will-return

This is normal Hindi order.

You can think of the basic pattern as:

• subject + object + time + verb

Of course, Hindi word order is flexible, but this placement is very natural.

Yes. That would also be natural.

For example:

• हालाँकि नियत तारीख कल है, फिर भी मैं उसे आज वापस करूँगी।

Both are understandable, but there is a slight difference in feel:

• लौटाऊँगी = return it, give it back
• वापस करूँगी = do/put it back, return it

In many everyday situations, especially with books or borrowed items, both work well.
लौटाऊँगी is a very direct verb for returning something.

The sentence itself is natural for anyone, but the exact verb form shows the speaker’s gender.

• female speaker: लौटाऊँगी
• male speaker: लौटाऊँगा

So the sentence as written specifically indicates that the speaker is female.

Everything else in the sentence stays the same:

• हालाँकि नियत तारीख कल है, फिर भी मैं उसे आज लौटाऊँगा।
  = same sentence, but said by a male speaker.