मैं किताब आज लौटाऊँगी क्योंकि नियत तारीख कल है।

लौटाऊँगी is the first-person singular future form of लौटाना, which means to return or to bring back something.

So here it means I will return.

A simple breakdown:

• लौटाना = to return something
• लौटाऊँगी = I will return, said by a female speaker

So the core of the sentence is:

• मैं किताब आज लौटाऊँगी = I will return the book today

The ending -ऊँगी shows two things at once:

• first person singular = I
• feminine speaker

So a female speaker says:

• मैं लौटाऊँगी = I will return

A male speaker would say:

• मैं लौटाऊँगा

This is one of the ways Hindi shows gender more clearly than English.

No. Here the verb is not feminine because of किताब. It is feminine because the speaker is female.

In this sentence, लौटाऊँगी matches मैं:

• female speaker: लौटाऊँगी
• male speaker: लौटाऊँगा

So the gender is about who is speaking, not about the noun किताब.

Because किताब is a direct object, and in Hindi, inanimate direct objects often appear without को.

So:

• मैं किताब लौटाऊँगी = normal and natural

Using को here is not impossible in every context, but it would usually sound less neutral or more marked. For a simple sentence like this, leaving out को is the most natural choice.

Yes, both are possible.

Hindi usually prefers the verb near the end, but the order of things before the verb is fairly flexible.

So these are both natural:

• मैं किताब आज लौटाऊँगी
• मैं आज किताब लौटाऊँगी

The difference is mainly one of emphasis or rhythm, not basic meaning.

A useful general pattern is:

• Subject + Object + Time + Verb but
• Subject + Time + Object + Verb is also very common.

The most important thing is that the verb लौटाऊँगी stays near the end.

क्योंकि means because.

It introduces the reason:

• मैं किताब आज लौटाऊँगी = I will return the book today
• क्योंकि नियत तारीख कल है = because the due date is tomorrow

So the full sentence means:

• I will return the book today, because the due date is tomorrow.

A very natural Hindi pattern is:

• main statement + क्योंकि + reason

You can also put the reason first, but then Hindi often likes इसलिए in the next clause:

• क्योंकि नियत तारीख कल है, इसलिए मैं किताब आज लौटाऊँगी।

Because है is being used for a present fact about a schedule.

In English, we also say:

• The due date is tomorrow not necessarily
• The due date will be tomorrow

So in Hindi:

• नियत तारीख कल है = The due date is tomorrow

This is a normal way to talk about dates, timetables, and scheduled events.

If you used होगी, it could sound more like a guess or assumption in many contexts:

• नियत तारीख कल होगी = The due date will probably be tomorrow / I suppose it is tomorrow

That is not the intended meaning here.

नियत means fixed, set, or appointed.
तारीख means date.

So नियत तारीख literally means something like:

• the fixed date
• the set date

In natural English, this is usually translated as:

• due date
• sometimes deadline, depending on context

It is understandable and correct Hindi, but it sounds a little formal. In everyday speech, people might also say things like:

• आख़िरी तारीख = last date / deadline
• लौटाने की तारीख = the date for returning
• sometimes even ड्यू डेट in casual modern speech

Because कल can mean either yesterday or tomorrow, and Hindi relies on context to tell you which one is meant.

Here the context makes tomorrow the only logical reading:

• मैं किताब आज लौटाऊँगी = I will return the book today
• क्योंकि नियत तारीख कल है = because the due date is tomorrow

If कल meant yesterday, the sentence would not make sense in the same way.

So this is a good example of how Hindi uses context to resolve ambiguity.

It could be left out if the context already makes the subject clear.

For example:

• किताब आज लौटाऊँगी क्योंकि नियत तारीख कल है।

That still means:

• I will return the book today because the due date is tomorrow.

Hindi often drops subjects when they are obvious from context, especially because the verb form already gives useful information. Here लौटाऊँगी already tells us the speaker is:

• first person
• singular
• feminine

Still, keeping मैं is perfectly natural and makes the sentence clearer, especially for learners or when the speaker wants to be explicit.

Yes, definitely.

A very natural alternative is:

• मैं किताब आज वापस करूँगी क्योंकि नियत तारीख कल है।

Both are correct.

A rough difference:

• लौटाना = to return, to bring back
• वापस करना = to give back, return

In everyday conversation, वापस करना is often very common.
For a library book, borrowed item, or object you are giving back, both work well.

Yes, it is correct and understandable, but नियत तारीख sounds a bit formal or written.

In everyday speech, many speakers might choose something slightly more conversational, such as:

• मैं किताब आज लौटाऊँगी क्योंकि कल आख़िरी तारीख है।
• मैं किताब आज वापस करूँगी क्योंकि कल लौटाने की तारीख है।

So the original sentence is good Hindi, but a native speaker in casual conversation might choose a less formal phrase for due date.