Hoy voy a arreglar la persiana, porque sin ella entra demasiada luz.

Voy a + infinitive is a very common way to talk about a planned or near-future action, similar to English I’m going to fix....
Arreglaré (simple future) is also correct, but it can sound more formal, more distant, or sometimes more like a promise/prediction depending on context. In everyday speech, voy a arreglar is often the default.

In this sentence, arreglar means to fix/repair (to make something work properly).
Depending on context, arreglar can also mean to tidy up/straighten up (e.g., arreglar la casa = tidy the house) or to arrange (in some contexts). But with something like la persiana, it’s clearly repair/fix.

Persiana usually refers to a window blind or rolling shutter (often slatted).
Usage can vary by country: in some places, people more commonly say cortina (curtain) or specify persiana enrollable (roller blind) or persiana metálica (metal shutter).
So persiana is correct Spanish, but depending on the region, another word might be more common for what you personally have at home.

Yes, persiana is grammatically feminine, so it takes feminine articles/adjectives: la persiana, una persiana, etc.
That’s why the sentence later uses ella to refer back to it.

Sin ella means without it, referring back to la persiana. Spanish often uses él/ella to refer to objects when you want a clear “it” that points back to a specific noun.
You could also say sin la persiana, but that repeats the noun.
Sin eso would mean without that, and it sounds less specific/more generic than referring directly to the blind.

Literally: too much light enters/comes in.
Luz is singular, so the verb is singular: entra.
Spanish commonly uses this “enter/come in” structure to talk about light/air/water, where English might say too much light comes in.

Because demasiado/a agrees in gender and number with the noun it modifies.
Luz is feminine singular → demasiada luz.
(Compare: demasiado ruido = too much noise, where ruido is masculine.)

Yes.

• mucha luz = a lot of light (could be neutral or just descriptive)
• demasiada luz = too much light (clearly a problem/excess)
  So demasiada emphasizes that the amount is more than desired.

It’s common to use a comma when porque introduces an explanatory reason after a complete main clause:
Hoy voy a arreglar la persiana, porque...
In informal writing, you may see it without the comma too. With the comma, the pause is clearer in writing, similar to English.

• porque (one word) = because (gives a reason)
• por qué (two words) = why (used in questions)
  This sentence gives a reason, so it must be porque. You’d use por qué only if you asked a question like: ¿Por qué vas a arreglar la persiana?

You can move it, and the sentence stays correct. Common options:

• Hoy voy a arreglar la persiana... (very natural)
• Voy a arreglar la persiana hoy... (also natural; slightly more emphasis on the action first)
  Spanish word order is flexible, and placement often changes what feels emphasized.

Yes, but the meaning shifts slightly:

• Entra demasiada luz focuses on light coming in from outside (through the window/space).
• Hay demasiada luz focuses on the situation inside: there is too much light (could be from lamps, sun, etc.).
  With la persiana, entra demasiada luz is especially logical.