wǒ dāngshí bù zài jiā.

Mandarin usually puts time expressions early in the sentence, often right after the subject.

So the pattern here is:

Subject + time + negation + verb/location phrase

• 我 = I
• 当时 = at that time / then
• 不在家 = not at home

So 我当时不在家 is a very natural Mandarin order.

English and Chinese organize information differently:

• English often puts then / at that time at the end.
• Chinese very often puts it near the beginning.

当时 means at that time, then, or at that moment, depending on context.

It refers to a specific time in the past that has already been mentioned or is understood from the situation.

Examples:

• 我当时不在家。 = I wasn’t at home at that time.
• 你当时在哪儿？ = Where were you at that time?

It is a bit more specific than simply saying before or in the past. It points to a particular moment or period.

Here, 在 works like a main verb meaning to be at / to be in / to be located at.

So:

• 我在家 = I am at home
• 我不在家 = I am not at home

In this sentence, 在 is not just marking location after another verb. It is the main idea: being somewhere.

This is different from a sentence like:

• 我在家吃饭。 = I eat at home

In that sentence, the first 在家 acts more like a location phrase before the main verb 吃.

But in 我当时不在家, 在 itself carries the meaning to be located.

This is a very common learner question.

For states, habits, and general situations, Mandarin usually uses 不 for negation.

Here, 在家 is describing a state/location: being at home.

So:

• 我不在家 = I am not at home / I was not at home

Using 没 is more closely connected with:

• completed actions
• have not
• past occurrence in many contexts

So:

• 我没吃饭。 = I didn’t eat / I haven’t eaten

But with 在 meaning to be located at, 不在 is the normal form.

Even if the sentence refers to the past because of 当时, Chinese still says:

• 我当时不在家

not usually:

• 我当时没在家

You may hear 没在家 in casual speech in some contexts, but for learners, 不在家 is the standard and safest form.

Mandarin does not usually need a special word equivalent to English was in this kind of sentence.

Chinese often does not mark tense directly the way English does. Instead, time is understood from:

• time words like 当时
• context
• aspect markers when needed

So:

• 我不在家 can mean I am not at home
• 我当时不在家 clearly means I was not at home at that time

The past meaning comes from 当时, not from changing the verb itself.

Because 是 is not used for location.

In English, we say:

• I am at home
• I was at home

So learners often want to use 是 for am/is/are/was/were.

But in Mandarin:

• 是 links nouns or noun phrases
• location is usually expressed with 在

Compare:

• 我是老师。 = I am a teacher.
• 我在家。 = I am at home.

So:

• 我当时不在家 is correct
• 我当时不是家 is incorrect

家 can mean different things depending on context:

• home
• family
• sometimes a measure-like suffix for establishments or experts in other words

In 在家, it means at home.

So:

• 我在家 = I’m at home

Even though 家 can also relate to family, here the meaning is definitely home / one’s residence.

This is a very common fixed expression:

• 在家 = at home
• 回家 = go home
• 离家 = leave home

In Chinese, when the meaning is obvious, speakers often do not say my home explicitly.

So:

• 在家 already naturally means at home
• it usually implies at one’s own home, unless context says otherwise

Saying 在我的家 is grammatically possible, but it sounds more literal or emphasized than normal everyday speech.

Most of the time, native speakers simply say:

• 我在家
• 他不在家
• 你今天在家吗？

Yes. This is a useful pronunciation point.

不 is normally fourth tone: bù.

But when 不 comes before another fourth-tone syllable, it is pronounced bú instead.

Since 在 is fourth tone (zài), 不在 is pronounced:

bú zài

So the sentence is pronounced: wǒ dāngshí bú zài jiā

This is a pronunciation change, not a spelling change in standard pinyin writing.

Yes, often you can.

Both can mean at that time / then, but they are not always identical in feeling.

• 当时 often sounds a bit more focused on a specific moment or situation already under discussion.
• 那时候 can feel a bit more like back then or at that time in a broader sense.

So these are both natural:

• 我当时不在家。
• 我那时候不在家。

In many everyday situations, either one works.

Because the sentence is just describing a past situation, not emphasizing a completed action or a change of state.

了 is not a general past-tense marker. Beginners often think it means past, but that is too simple.

Here, the sentence means:

• At that time, I was not at home

The time is already clear from 当时, so no 了 is needed.

If you added 了, the meaning would usually change and might sound like a change of situation rather than a simple description.

So the plain sentence without 了 is the natural choice.

Yes. Those are just two natural English ways to express the same basic idea.

In Mandarin, 我当时不在家 is compact and flexible. Depending on context, it could be translated as:

• I wasn’t home then.
• I wasn’t at home at that time.
• I was not at home then.

The exact English wording can vary, but the Chinese sentence stays the same.

It is a complete and natural sentence on its own.

However, 当时 usually points to a time already known in the conversation, so in real use it often appears with context.

For example:

• 你怎么没接电话？
• 我当时不在家。

Even without extra context, though, the sentence is fully grammatical and natural.