Learn Hardcore Japanese: kaaten wo aketara, heya ga akaruku narimasita. - When I opened the curtains, the room became bright.

カーテンを開けたら: 開ける is a transitive verb (“to open something”), so its direct object (カーテン) is marked with を.
部屋が明るくなりました: 明るくなる is an intransitive change-of-state verb (“to become bright”), so the thing that changes (the room) is marked with が.

What nuance does the ～たら form add here? Isn’t that an “if” conditional?

The ～たら form can express both “if” and “when/after” in Japanese.
In past-tense contexts like 開けたら, it often means “when/after I opened… then…” rather than a hypothetical “if.”
Compared with ～と, ～たら allows more flexibility:
• ～たら can show unexpected results and works with past events.
• ～と often expresses a natural or guaranteed result as soon as one action occurs.

Why isn't it 開けましたら instead of 開けたら, since the main clause uses polite なりました?

In Japanese subordinate clauses (like conditionals), you always use the plain form.
Politeness (–ます, –ました) is marked on the main clause only.
So even in polite speech you get 開けたら部屋が明るくなりました, not 開けましたら.

Why do we say 明るくなる instead of 明るいなる?

明るい is an i-adjective. To connect it to なる (“to become”), you convert 明るい → 明るく (adverbial form).
Then 明るく
- なる literally means “become bright.”
You never say 明るいなる—the い must change to く when linking to なる.

Who is doing the action in each clause? Why is the subject omitted in the first clause?

Japanese often drops subjects when they’re obvious from context.
開けたら implies “I opened…” or “we opened…” even though 私が is not said.
In the second clause the new subject is explicit: 部屋が “the room” is what became bright.

How would I say “The room was bright” rather than “became bright”?

To describe a state (“was bright”), use the past form of the adjective:
• Polite: 部屋が明るかったです
• Plain: 部屋が明るかった
明るくなりました means “became bright,” emphasizing a change rather than a static property.