atarasii syoumei ni kaetara, heya ga mae yori akaruku narimasita.

With 替える meaning to replace / to switch to, the thing you switch to is often marked with に.

So in a fuller version, you could say:

• 照明を新しい照明に替える = replace the lighting with new lighting / switch the lighting to new lighting

In your sentence, the old thing being replaced is omitted because it is understood from context. So:

• 新しい照明に替えたら = when/after I switched to new lighting

The に marks the resulting state or new item.

たら can mean if, when, or after, depending on context.

Here, it describes something that happened first, followed by a result:

• 新しい照明に替えたら、部屋が前より明るくなりました。
• After I changed to new lighting, the room became brighter than before.

So in this sentence, たら is not really a hypothetical if. It is more like:

• when I changed it
• after changing it

It often introduces a discovered or resulting situation.

Because 明るい is an い-adjective, and before なる it changes to its -く form.

Pattern:

• い-adjective → く + なる

Examples:

• 明るい → 明るくなる = become bright/brighter
• 寒い → 寒くなる = become cold/colder
• 高い → 高くなる = become expensive/higher

So:

• 部屋が明るくなりました = the room became bright/brighter

You cannot say 明るいになる.

より means than in comparisons.

So:

• 前より = than before
• 去年より = than last year
• これより大きい = bigger than this

In your sentence:

• 部屋が前より明るくなりました
• The room became brighter than before

It compares the room’s current brightness with its earlier brightness.

Japanese often uses short expressions when the meaning is clear from context.

Here, 前 simply means before / earlier / previously, and that is enough.

So:

• 前より = than before

It is a very natural, compact way to compare the current situation with the previous one.

You could say more explicitly things like:

• 前の状態より
• 以前より

But 前より sounds natural and common in everyday speech.

In this sentence, 部屋 is the thing that undergoes the change:

• 部屋が明るくなりました = the room became brighter

The particle が marks the subject of なる here.

Using が makes sense because the sentence is describing what changed as a result of the lighting replacement.

You could also sometimes see 部屋は, but the nuance changes:

• 部屋が明るくなりました = focuses on the room became brighter
• 部屋は明るくなりました = makes the room the topic, possibly with contrast

In this sentence, が is very natural.

Both can be read かえる, but the kanji suggest different meanings.

• 替える = replace / exchange / switch one thing for another
• 変える = change / alter / modify

In this sentence, 替える is used because the lighting was replaced with different lighting, not just changed in an abstract way.

Examples:

• 電池を新しいものに替える = replace the batteries with new ones
• 予定を変える = change the plan
• 考え方を変える = change your way of thinking

So 替える fits the idea of swapping the old lighting for new lighting.

照明（しょうめい） usually means lighting or illumination in a general sense, rather than one specific bulb or lamp.

Depending on context, it could refer to:

• a lighting fixture
• the lighting setup
• the room’s lighting

So 新しい照明に替えたら most naturally means something like:

• when I switched to new lighting
• when I replaced the light fixture / lighting setup

If you wanted to talk specifically about a light bulb, you would more likely use words like:

• 電球
• 蛍光灯
• ライト

Japanese often omits subjects when they are obvious from context.

So instead of saying:

• 私は新しい照明に替えたら…

the sentence simply says:

• 新しい照明に替えたら…

The listener naturally understands that someone relevant—probably I or we—made the change.

This is extremely common in Japanese and does not sound incomplete.

Because adjectives in Japanese usually come before the noun they modify.

So:

• 新しい照明 = new lighting
• 明るい部屋 = bright room
• 大きい家 = big house

This works much like English adjective + noun order.

It can mean either, depending on context.

By itself:

• 明るくなりました = became bright / became brighter

In this sentence, because of 前より (than before), the meaning is clearly comparative:

• became brighter than before

So the comparison comes from 前より, not from the form 明るくなりました alone.

The past tense shows that both events are being described as completed:

1. the lighting was changed
2. the room became brighter

• 替えたら contains the past form 替えた
• なりました is the polite past of なる

This gives the sense of a completed action and its result.

Yes. That is a very natural translation.

Depending on context, all of these are reasonable:

• When I changed to new lighting, the room became brighter than before.
• After I replaced the lighting, the room got brighter.
• Once I switched to new lighting, the room was brighter than before.

The exact English wording can vary, but the Japanese structure stays the same.

The comma separates the condition/time clause from the main result clause:

• 新しい照明に替えたら、 = when/after I changed to new lighting,
• 部屋が前より明るくなりました。 = the room became brighter than before.

Japanese commas are somewhat flexible, but here the comma makes the sentence easier to read and clearly separates the two parts.

Not if you want to keep the meaning than before.

• 前より明るくなりました = became brighter than before
• 前、明るくなりました would not mean the same thing and would sound wrong for this comparison

So より is necessary here because it is what creates the comparison.

Because the sentence uses ました, the polite past form.

• なる = dictionary form
• なった = plain past
• なりました = polite past

So a plain-style version would be:

• 新しい照明に替えたら、部屋が前より明るくなった。

Your sentence is simply the polite version, which is common in standard conversation and writing for learners.

Yes, that nuance is possible.

Japanese たら often introduces a result that became clear after doing something. So this sentence can carry a feeling like:

• When I changed to new lighting, it turned out the room became brighter than before.

That discovery nuance is subtle, but it fits very naturally with たら.

It relates to the degree of brightness, so it goes with 明るくなりました.

In other words, the meaning is:

• The room became brighter, compared with before

It does not mean something like the earlier room.
It is comparing the room’s current brightness to its previous brightness.

A helpful breakdown is:

• [new thing] に 替える = switch to / replace with [new thing]
• V-たら = when / after / if doing V
• [noun] が [い-adjective]-く なる = [noun] becomes [adjective]

So the sentence follows this structure:

• 新しい照明に替えたら、部屋が前より明るくなりました。
• After switching to new lighting, the room became brighter than before.

This combines:

1. a replacement action
2. a たら clause
3. a change-of-state expression with なる