Learn Hardcore Japanese: hiru ha mado wo akereba, denki wo tukenakute mo heya ga akaruku miemasu. - During the day, if I open the window, the room looks bright even without turning on the light.

In Japanese, は marks the topic rather than the grammatical subject. By saying 昼は, you’re setting “daytime” as the context or topic: “As for daytime...” In English we might translate this as “During the day...”

What does the ば ending in 開ければ indicate?

開ければ is the -ば conditional form of the verb 開ける (“to open”). It means “if you open”. The -ば conditional expresses a general condition: “If/when you open the window...”

Could I use 開けたら or 開けると instead of 開ければ? Would it change the nuance?

Yes. These are different conditional patterns:

開けたら (the -たら conditional) is often used for specific or sequential events.
開けると implies an inevitable or natural result when one action follows another.
開ければ is more general and neutral.
In this sentence, all three would be understood, but 開ければ emphasizes a general condition rather than a single occurrence or a natural sequence.

Why is 電気をつけなくても used to express “without turning on the lights,” and what’s the role of ても?

This comes from the negative -ない form plus -ても, meaning “even if [you] don’t…”. So 電気をつけなくても literally means “even if you don’t turn on the light,” which naturally translates as “without having to turn on the light.”

Why is 部屋 marked with が rather than を in 部屋が明るく見えます?

In the phrase 部屋が明るく見えます, 部屋 is the subject of the intransitive verb 見える (“to appear/to be visible”), so it takes が. The particle を would only be used if 部屋 were the direct object of a transitive verb.

Why is 明るく used instead of 明るい before 見えます?

明るい is an i-adjective meaning “bright.” To modify a verb (見えます), you use the adverbial form (連用形), which for i-adjectives ends in -く. Hence 明るく見えます = “appears bright.”

What’s the difference between 見える and 見る, and why is 見える used here?

見る is a transitive verb meaning “to see; to look (at something).”
見える is intransitive and means “to be visible; to appear.”
Since the sentence describes the room’s appearance (“the room looks bright”), 見える is the correct choice.

There’s no explicit subject like “I” or “you.” Who is doing the opening and seeing here?

Japanese often omits pronouns when they’re clear from context. Here it’s describing a general or impersonal situation: “If (one) opens the window during the day, the room looks bright even without turning on the light.” The subject is implicitly anyone.