wǒ chūmén qián wàng le guān dēng.

Because Chinese often forms before/after doing something expressions as:

verb phrase + 前 / 后

So:

• 出门前 = before going out
• 吃饭前 = before eating
• 睡觉前 = before sleeping

In other words, 前 means before, but it comes after the action it refers to.

So 我出门前忘了关灯 literally follows the pattern:

I + before going out + forgot + turn off the light

That is normal Chinese word order.

Chinese usually puts time expressions before the main action.

A very common pattern is:

subject + time expression + verb phrase

So here:

• 我 = subject
• 出门前 = time expression
• 忘了关灯 = main event

This is why the sentence is arranged as:

我 出门前 忘了 关灯

That word order is much more natural in Chinese than putting the time phrase later.

Here, 了 marks a completed event.

So 忘了 means forgot or ended up forgetting.

It does not mean past tense in exactly the same way as English. Instead, it shows that the action/state of forgetting happened as a completed event.

Compare:

• 我忘了关灯。 = I forgot to turn off the light.
• 我忘关灯。 = grammatically possible in some contexts, but sounds less natural and less complete on its own.

In this sentence, 了 makes the statement sound natural and finished.

Because 忘了关灯 means forgot to turn off the light.

The structure is:

忘了 + action

So:

• 忘了带手机 = forgot to bring my phone
• 忘了锁门 = forgot to lock the door
• 忘了关灯 = forgot to turn off the light

If you say 关了灯, that means turned off the light as a completed action. Putting that after 忘 changes the logic and usually does not express the same idea.

So:

• 忘了关灯 = I failed to do it because I forgot.
• 忘了关了灯 = awkward / usually not what you want.

Yes. In Chinese, 关 is used not only for closing things, but also for switching off many devices.

So:

• 关灯 = turn off the light
• 关门 = close the door
• 关电视 = turn off the TV
• 关空调 = turn off the air conditioner

The opposite is usually 开:

• 开灯 = turn on the light
• 开电视 = turn on the TV

So even though 关 is often taught as close, in many everyday situations it naturally means turn off.

Yes. 忘 and 忘记 are both common, and both can work here.

For example:

• 我出门前忘了关灯。
• 我出门前忘记了关灯。

Both are correct.

In general:

• 忘 is shorter and very common in speech.
• 忘记 is slightly fuller or a bit more formal, but still very natural.

So learners should feel comfortable with either one.

Because the subject is already understood to be the same person.

In 我出门前忘了关灯, the person who was going out and the person who forgot to turn off the light are both 我.

Chinese often omits repeated subjects when they are clear from context.

So Chinese does not need something like:

我出门前忘了我关灯

That would be unnatural.

The meaning is simply understood as:

Before I went out, I forgot to turn off the light.

After 忘 / 忘了, Chinese often puts the action that was forgotten.

So 关灯 is the action that was forgotten.

Pattern:

忘了 + verb phrase

Examples:

• 忘了带钥匙 = forgot to bring the keys
• 忘了写作业 = forgot to do the homework
• 忘了回电话 = forgot to return the call
• 忘了关灯 = forgot to turn off the light

So 关灯 is not a separate sentence here. It is the action serving as the content/object of 忘了.

They are related, but not the same.

• 我出门前忘了关灯。 = I forgot to turn off the light before going out.
• 我出门前没关灯。 = I didn’t turn off the light before going out.

The second sentence only states the fact that the light was not turned off. It does not explain why.

The first sentence specifically says the reason was forgetting.

So:

• 没关灯 = didn’t turn it off
• 忘了关灯 = forgot to turn it off

Yes, you often can.

Both of these can mean before going out:

• 出门前
• 出门以前

In this sentence, 出门前 is the more compact and very natural choice.

A simple way to think about it:

• 前 often attaches directly to an action: 吃饭前, 睡觉前, 出门前
• 以前 also means before, and can be a little broader or more explicit

So 我出门前忘了关灯 is probably the most natural everyday version, but 我出门以前忘了关灯 is also understandable and acceptable.

Yes, but it changes the feeling a little.

• 我出门前忘了关灯。 = neutral statement
• 我出门前忘了关灯了。 = often sounds more like a realization, new situation, or emotional notice

The final 了 can suggest something like:

• Oh no, I forgot to turn off the light before I left.
• Now I realize I forgot to turn off the light.

So the version with one 了 is simpler and more neutral. The version with two 了 is possible, but more context-dependent.