أنا في البيت في الليل.

In Arabic, the verb to be is usually not written in the present tense.

So instead of saying something equivalent to I am in the house, Arabic simply says:

أنا في البيت literally: I in the house

This is completely normal in Modern Standard Arabic.
If you wanted the past or future, then Arabic would use a verb, but in the present it is usually omitted.

Yes, often you can.

Arabic commonly allows the subject pronoun to be dropped when it is understood from context. So:

في البيت في الليل

could be understood as in the house at night, and depending on context it may imply I am in the house at night.

However, keeping أنا can be useful:

• for emphasis
• for clarity
• for beginner-friendly full sentences

So أنا في البيت في الليل is perfectly fine, even if أنا is not always necessary in real usage.

Because each في introduces its own phrase:

• في البيت = in the house
• في الليل = at night / in the night

Arabic often repeats the preposition where English might not think much about it. Here, each phrase is separate, so each one takes في.

This is natural Arabic.

البيت means the house / the home, while بيت means a house / house.

The prefix الـ is the Arabic definite article, meaning the.

So:

• بيت = a house / house
• البيت = the house / the home

In many contexts, البيت can also mean home, not just a physical house.

Both are possible, but they mean slightly different things.

• ليل = night, nighttime, night in a general sense
• الليل = the night, the nighttime

In Arabic, time expressions often appear with الـ, so في الليل is a very common way to say at night.

So even though English says at night without the, Arabic often says في الليل, literally in the night.

This is a great pronunciation question.

In الليل, the ل of the article الـ is assimilated because the next letter, ل, is a sun letter.

So although it is written:

الليل

it is pronounced more like:

al-layl
with a doubled l sound

The same does not happen in البيت, because ب is a moon letter. So:

• البيت = al-bayt
• الليل = al-layl with a stronger/doubled l

This is a nominal sentence in Arabic, meaning it starts without a verb.

You can break it down like this:

• أنا = the subject
• في البيت = a prepositional phrase
• في الليل = another prepositional phrase

The first prepositional phrase, في البيت, functions as the predicate, meaning it tells us something about أنا.

So the structure is roughly:

• أنا = topic / subject
• في البيت = predicate
• في الليل = time expression

This is a very common structure in Arabic.

It can change somewhat.

The basic sentence:

أنا في البيت في الليل

is clear and correct.

But Arabic allows some flexibility for emphasis. For example:

• أنا في البيت في الليل = neutral
• في الليل أنا في البيت = emphasis on at night
• في البيت أنا في الليل = possible, but less neutral and more context-dependent

For learners, the original order is a good standard order to use.

In full formal Arabic, yes, there can be case endings.

The fully inflected form would often be pronounced something like:

أنا في البيتِ في الليلِ

Why?

Because after the preposition في, nouns are in the genitive case, so they take -i in careful formal pronunciation:

• في البيتِ
• في الليلِ

In everyday speech and in most modern spoken reading, these short case endings are often not pronounced.

So beginners usually see and say:

أنا في البيت في الليل

and that is completely normal for learning.

A careful pronunciation is:

anā fī l-bayti fī l-layli

A more learner-friendly pronunciation, without stressing full case endings, is:

anā fī al-bayt fī al-layl

A more natural connected pronunciation often sounds like:

anā fil-bayt fil-layl

A few notes:

• أنا = anā
• في = fī
• البيت = al-bayt
• الليل = al-layl with a doubled l feel

Yes, it is very common and natural in Modern Standard Arabic.

Arabic often uses في with times and periods like this. So:

• في الليل = at night
• في الصباح = in the morning
• في المساء = in the evening

So even if English uses different prepositions like at or in, Arabic often uses في.

Yes, very possibly.

البيت can mean:

• the house
• the home
• home

So depending on context, أنا في البيت في الليل could be understood as:

• I am in the house at night
• I am at home at night

That is one reason بيت is such a useful word.

Not necessarily.

In Modern Standard Arabic, the sentence is correct and natural. But in everyday spoken Arabic, many speakers might use dialect forms instead.

For example, in dialects you may hear different words or shortened forms. But for Modern Standard Arabic, this sentence is a perfectly good model.

So for learning MSA, it is absolutely worth understanding and using this structure.