ie wo deru mae ni, doa ni kagi wo kakemasu.

Because 出る is modifying 前 in the pattern Verb + 前に, which means before doing ....

When a verb comes before a noun-like word such as 前, it normally appears in its plain form:

• 家を出る前に = before leaving the house

Using 出ます前に would be ungrammatical.

This is similar to how plain-form verbs are used before nouns in other structures too.

Dictionary form verb + 前に means before doing something.

So:

• 家を出る前に = before leaving the house

A useful comparison:

• 食べる前に = before eating
• 寝る前に = before sleeping
• 行く前に = before going

The verb stays in the dictionary form because the action has not happened yet.

In 前に, the に marks the time point before which the main action happens.

So the sentence structure is:

• 家を出る前に = before leaving the house
• ドアに鍵をかけます = I lock the door

Together: Before leaving the house, I lock the door.

Sometimes 前 can appear without に, but 前に is the standard form in this grammar pattern.

Because with verbs of movement like 出る, the particle を can mark the place that you leave or pass through.

So:

• 家を出る = leave the house
• 部屋を出る = leave the room
• バスを降りる = get off the bus

This is a very common use of を that surprises English speakers, because it is not a direct object in the English sense.

Yes, 家から出る is possible, but 家を出る is more natural and common when talking about physically exiting a place.

Roughly speaking:

• 家を出る = leave the house / go out of the house
• 家から出る = go out from the house

から emphasizes the starting point, while を is the usual particle with 出る for the place exited.

Because かける in this expression works like this:

• 鍵をかける = lock / apply a lock
• ドアに鍵をかける = lock the door

Here:

• 鍵 is marked by を because it is the thing being applied
• ドア is marked by に because it is the target

So literally, it is something like:

• put a lock on the door

That is why Japanese uses に with ドア here.

Yes, as a set expression, 鍵をかける means to lock.

Literally, かける has many meanings, such as to hang, to put on, or to apply, depending on the context. In this expression, it means something like to put the lock on.

Common pairs:

• 鍵をかける = lock
• 鍵を開ける = unlock / open the lock

So even though かける has many meanings, 鍵をかける should be learned as one chunk.

They are doing completely different jobs.

1. 前に

   • marks a time relation: before

2. ドアに

   • marks the target of the locking action: on/to the door

So even though both are に, their meanings come from the grammar pattern they are part of.

The comma is there to separate the time clause from the main clause, making the sentence easier to read.

• 家を出る前に、 = before leaving the house,
• ドアに鍵をかけます。 = I lock the door.

It is helpful, but not always absolutely required in casual writing. Japanese commas are often used more flexibly than English commas.

Yes. Japanese word order is flexible as long as the particles make the relationships clear.

For example, these are all understandable:

• 家を出る前に、ドアに鍵をかけます。
• ドアに鍵をかけます、家を出る前に。
• 家を出る前に、鍵をドアにかけます。

However, the original sentence sounds the most natural because it presents the time setting first, then the main action.

Also, 鍵をドアにかけます is grammatical, but ドアに鍵をかけます sounds more standard in this context.

Yes. 前 is basically a noun meaning front or before.

That is why it can be used in structures like:

• 駅の前 = in front of the station
• 食べる前に = before eating

In 家を出る前に, the whole clause 家を出る modifies 前, almost like saying:

• the before of leaving the house

Of course, in natural English we just say before leaving the house.

They are:

• 家 = いえ
• を = o
• 出る = でる
• 前 = まえ
• に = ni
• ドア = doa
• 鍵 = かぎ
• かけます = kakemasu

So the full sentence is read:

いえ を でる まえ に、ドア に かぎ を かけます。

Sometimes, yes, depending on the exact meaning.

• 家を出る前に = before leaving the house
• 出かける前に = before going out / before heading out

出かける often implies going out somewhere, not just stepping outside.
家を出る focuses more literally on leaving the house.

So if the idea is simply before I leave home, the original sentence is perfectly natural. If the idea is before I go out, 出かける前に might also work.