मैं सुबह तैयार रहूँगी, क्योंकि मुझे ऑफिस जाना है।

रहूँगी comes from रहना (to remain / stay / be), and in sentences like this it often means will be in a state.

So:

• तैयार रहूँगी = I will be ready
• literally, I will remain/stay ready

This is very natural in Hindi.
If you said तैयार होऊँगी, it would sound more like I will become ready / I will get ready, not simply I will be ready.

So the difference is roughly:

• तैयार रहूँगी = I will be ready
• तैयार होऊँगी = I will get/become ready

The ending -ूँगी marks:

• future tense
• first person singular (I)
• feminine speaker

So रहूँगी means I will stay/be when the speaker is female.

Compare:

• मैं ... रहूँगी = I will ... (female speaker)
• मैं ... रहूँगा = I will ... (male speaker)

So if a man said this sentence, it would be:

• मैं सुबह तैयार रहूँगा, क्योंकि मुझे ऑफिस जाना है।

तैयार is an adjective meaning ready, and many Hindi adjectives do not change form for gender or number.

So:

• मैं तैयार हूँ = I am ready
• वह तैयार है = He/She is ready
• हम तैयार हैं = We are ready

This is unlike changeable adjectives such as:

• अच्छा (masculine singular)
• अच्छी (feminine singular)
• अच्छे (plural / masculine oblique)

But तैयार stays the same.

सुबह means morning, and depending on context it can mean:

• in the morning
• tomorrow morning
• this morning (if the context allows)

In this sentence, because रहूँगी is future, सुबह is understood as a future morning time, often naturally tomorrow morning or simply in the morning.

Hindi often leaves this kind of time reference to context instead of making it completely explicit.

मुझे ऑफिस जाना है is a very common Hindi structure for expressing have to / need to / am supposed to.

Structure:

• मुझे = to me / I
• ऑफिस जाना = going to the office
• है = is

Literally, it is something like:

• To me, going to the office is required

Natural English:

• I have to go to the office

So Hindi often uses this pattern:

• मुझे जाना है = I have to go
• उसे पढ़ना है = He/She has to study
• हमें निकलना है = We have to leave

This is why मुझे is used instead of मैं.

Not always. Grammatically, मुझे is related to I/me, but in actual usage it can mean different things depending on the construction.

For example:

• मुझे पता है = I know
• मुझे भूख लगी है = I am hungry
• मुझे जाना है = I have to go
• मुझे वह पसंद है = I like that

So although it comes from the oblique/dative form of मैं, you should often translate it naturally, not literally.

In this sentence:

• मुझे ऑफिस जाना है = I have to go to the office

not awkwardly To me, office-going is.

क्योंकि means because.

It connects the two parts:

• मैं सुबह तैयार रहूँगी = I will be ready in the morning
• क्योंकि मुझे ऑफिस जाना है = because I have to go to the office

So the full sentence means:

• I will be ready in the morning, because I have to go to the office.

A useful thing to know: Hindi also often uses क्योंकि ... इसलिए ... as a pair:

• क्योंकि मुझे ऑफिस जाना है, इसलिए मैं सुबह तैयार रहूँगी।
• Because I have to go to the office, therefore I will be ready in the morning.

But in everyday speech, using only क्योंकि is completely normal too.

Yes, Hindi word order is fairly flexible, though some orders sound more natural than others.

Original:

• मैं सुबह तैयार रहूँगी, क्योंकि मुझे ऑफिस जाना है।

Also natural:

• क्योंकि मुझे ऑफिस जाना है, मैं सुबह तैयार रहूँगी।

You could also hear:

• मुझे ऑफिस जाना है, इसलिए मैं सुबह तैयार रहूँगी।

What usually stays important is that the verb comes toward the end of each clause.

ऑफिस is a very common borrowed word from English, and it is widely used in everyday Hindi.

You may also see:

• दफ़्तर = office

So these are both possible:

• मुझे ऑफिस जाना है
• मुझे दफ़्तर जाना है

In modern spoken Hindi, especially in cities, ऑफिस is extremely common.

Yes. Hindi allows some flexibility with time words.

For example:

• मैं सुबह तैयार रहूँगी = I will be ready in the morning
• मैं तैयार रहूँगी सुबह = possible, but less neutral and more marked/emphatic

The most natural neutral position is usually before the adjective/verb phrase:

• मैं सुबह तैयार रहूँगी

That is the most standard way to say it.

Only the future verb would change:

• मैं सुबह तैयार रहूँगा, क्योंकि मुझे ऑफिस जाना है।

So:

• रहूँगी = female speaker
• रहूँगा = male speaker

The rest of the sentence stays the same.

Yes, often it can be omitted if the context already makes the subject clear.

For example:

• सुबह तैयार रहूँगी, क्योंकि मुझे ऑफिस जाना है।

This still clearly means:

• I’ll be ready in the morning, because I have to go to the office.

Hindi frequently drops pronouns when they are understood from context or from the verb ending.

Here, रहूँगी already tells you the speaker is I and female, so मैं is not always necessary.

It is neutral and natural everyday Hindi.

Nothing in it is especially formal or especially casual:

• मैं = normal I
• क्योंकि = standard because
• मुझे ... जाना है = standard everyday expression
• ऑफिस = common modern spoken word

So this sentence would fit normal conversation very well.