soto de kaminari ga natte iru node, mado wo simemasu.

で marks the place where something happens.

So in 外で, the meaning is outside / outdoors as the location of the action or event.

• 外で雷が鳴っている = Thunder is sounding outside
• Here, で tells you where the thunder is being heard.

This is different from に, which often marks existence or destination rather than the location of an event.

For example:

• 外に出ます = go outside
• 外で遊びます = play outside

In this sentence, thunder is an event happening outside, so で is natural.

が marks 雷 as the subject of the verb 鳴っている.

So:

• 雷が鳴っている = thunder is rumbling / there is thunder

Using が here presents the thunder as the thing currently happening or being noticed.

If you used は, it would sound more like you were making thunder the topic and saying something about it, such as contrasting it with something else.

Compare:

• 雷が鳴っている = There’s thunder rumbling
• 雷は鳴っている = As for the thunder, it is rumbling
  (more contrastive or topic-focused)

In this sentence, が is the most natural choice because the thunder is simply the reason for closing the window.

鳴っている is the ている form of 鳴る.

• 鳴る = to sound, to ring, to rumble
• 鳴っている = is sounding, is rumbling

In this sentence, it describes an ongoing state or event:

• 雷が鳴っている = thunder is rumbling

With weather and sounds, ている often shows that something is currently going on.

So this is not just thunder sounded once. It suggests that the thunder is happening now, which gives the reason for the next action: closing the window.

鳴る is a u-verb ending in る, and its て-form is 鳴って.

The pattern is:

• 〜る / う / つ → って

So:

• 鳴る → 鳴って
• 待つ → 待って
• 買う → 買って

Then:

• 鳴っている = 鳴って
  • いる

That gives the progressive/ongoing meaning: is sounding.

Both ので and から can mean because, but they have slightly different nuances.

• ので sounds a bit more neutral, explanatory, and softer
• から can sound more direct, personal, or assertive

So:

• 外で雷が鳴っているので、窓を閉めます。
  = Since thunder is rumbling outside, I’ll close the window.

This sounds like a calm explanation of the reason.

If you said:

• 外で雷が鳴っているから、窓を閉めます。

that would also be correct, but slightly more straightforward and conversational.

A very rough feeling is:

• ので = because / since in a more explanatory way
• から = because in a more direct way

In Japanese, the non-past form covers both:

• present/future actions
• habitual actions

So 閉めます here means something like:

• I will close the window
• I’m going to close the window

Japanese does not need a separate future tense the way English often does.

So:

• 窓を閉めます = I close / will close the window

In this context, because it follows a reason, it is understood as an action the speaker is about to do.

Japanese often leaves out subjects when they are obvious from context.

So 窓を閉めます does not explicitly say:

• I will close the window
• we will close the window
• someone will close the window

But in a normal situation, the listener naturally understands that the speaker is the one who will do it.

This omission is very common in Japanese. If the subject is clear, it is usually not stated.

So the full idea is something like:

• Because thunder is rumbling outside, I’ll close the window.

Even though I is not actually spoken.

This is the difference between a transitive and an intransitive verb.

• 閉める = to close something
• 閉まる = to close / be closed by itself

So:

• 窓を閉めます = I close the window
• 窓が閉まります = The window closes / becomes closed

In your sentence, the speaker is actively doing the action, so the transitive verb 閉める is needed.

That is why the object takes を:

• 窓を閉めます

Sometimes learners wonder about 閉じる because it also relates to closing.

In this sentence, 閉めます is the most natural choice for closing a window.

A simple way to think about it:

• 閉める is commonly used for things like windows, doors, shops, curtains, and eyes
• 閉じる often means to shut/close too, but is frequently used in slightly different contexts, such as books, files, or things that fold or come together

Examples:

• 窓を閉める = natural
• ドアを閉める = natural
• 本を閉じる = natural

There is some overlap, but for windows, 閉める is the standard choice.

The comma shows a pause between the reason and the main action.

• 外で雷が鳴っているので、窓を閉めます。

It helps the reader see the structure clearly:

1. Reason: thunder is rumbling outside
2. Result/action: I’ll close the window

Japanese commas are somewhat flexible, so you may also see similar sentences without a comma. But in a sentence like this, the comma is very natural and makes it easier to read.

Yes, 鳴ってる is a casual spoken contraction of 鳴っている.

• 鳴っている = more standard/full form
• 鳴ってる = casual speech

So:

• 雷が鳴っている
• 雷が鳴ってる

both mean the same thing, but the second is more conversational.

In writing, textbook examples, and polite sentences like this one, 鳴っている is more appropriate.

The readings are:

• 外 → そと
• 雷 → かみなり
• 鳴っている → なっている
• 窓 → まど
• 閉めます → しめます

So the full sentence is read:

そと で かみなり が なって いる ので、まど を しめます。

This is a useful sentence because it includes several very common words and patterns:

• place + で
• subject + が
• ている
• reason with ので
• object + を
• polite verb 〜ます