wǒ zuówǎn zài jiā.

Because Mandarin does not use a verb like English to be for location in this kind of sentence.

In 我昨晚在家, the word 在 is doing the job of be at / be in / be located at.

So:

• 我 = I
• 昨晚 = last night
• 在家 = at home

A very natural English translation is I was at home last night, even though there is no separate word meaning was.

Here, 在 means to be at, to be in, or to be located at.

It is important not to confuse this with another common use of 在, where it marks an ongoing action, as in 我在吃饭 (I am eating).

In 我昨晚在家, 在 is the main verb of the sentence:

• 我在家 = I am at home
• 我昨晚在家 = I was at home last night

Because 是 is generally not used to say where someone or something is.

English says:

• I am at home

But Mandarin does not normally say:

• 我是家 or 我是在家 for basic location

Instead, Mandarin uses 在:

• 我在家 = I’m at home

A simple rule:

• Use 是 for identity or classification: 我是老师 = I am a teacher
• Use 在 for location: 我在家 = I am at home

Mandarin usually puts time expressions before the main verb or before the main action/location phrase.

A common pattern is:

Subject + Time + Verb/Location

So:

• 我 + 昨晚 + 在家

This is very natural Mandarin word order.

English often puts time at the end:

• I was at home last night

Mandarin often moves the time earlier:

• 我昨晚在家

Yes. 昨晚我在家 is also correct and natural.

Both are common:

• 我昨晚在家
• 昨晚我在家

The difference is mainly emphasis or flow:

• 我昨晚在家 starts with I
• 昨晚我在家 starts with last night

So if you want to emphasize the time, putting 昨晚 first can feel slightly more natural.

Because in Mandarin, 在家 is a very common and natural way to say at home.

You can also say:

• 在家里

Both are correct:

• 我昨晚在家
• 我昨晚在家里

在家 is a bit shorter and very common in everyday speech.
在家里 sounds slightly more explicit, like at home / in the house.

It most directly means I was at home last night.

Depending on context, English might also translate it as:

• I stayed home last night

But grammatically, the Chinese sentence is simply stating location: I was at home.

If you want to clearly express the idea of stayed home, Mandarin often uses a verb such as:

• 我昨晚待在家里 = I stayed at home last night

So 我昨晚在家 is simpler and more neutral.

Because Mandarin does not always need a special past marker.

The time word 昨晚 already makes it clear that this happened in the past:

• 昨晚 = last night

So adding 了 is unnecessary here.

Mandarin often shows past time through time expressions rather than verb changes:

• 我昨天很忙 = I was very busy yesterday
• 他去年在北京 = He was in Beijing last year

No verb conjugation is needed.

Sometimes, yes, if the context already makes it obvious who you mean.

For example, in conversation:

• 你昨晚在哪儿？ = Where were you last night?
• 昨晚在家。 = At home last night.

That is natural in speech.

But as a full standalone sentence, 我昨晚在家 is the clearer and more complete version.

Yes, it is complete.

In Mandarin, being in a place can itself be the main idea of the sentence. The location verb 在 is enough.

So 我昨晚在家 is a full sentence meaning that your location last night was home.

You do not need to add another verb unless you want a more specific meaning, such as:

• 我昨晚待在家 = I stayed at home last night
• 我昨晚回家了 = I went home last night
• 我昨晚住在家里 = I lived/stayed at home last night

They are very close in meaning.

• 昨晚 = last night
• 昨天晚上 = yesterday evening / last night

In many situations, they can be used interchangeably:

• 我昨晚在家
• 我昨天晚上在家

昨晚 is shorter and very common.
昨天晚上 is a little fuller and slightly more explicit.

It is very natural.

我昨晚在家 is a simple, normal sentence that a native speaker could easily say in conversation.

It works especially well when answering a question like:

• 你昨晚在哪儿？ = Where were you last night?

Answer:

• 我昨晚在家。

So this is not a textbook-only sentence; it is real everyday Mandarin.