Si apago la televisión, puedo concentrarme mejor en mis tareas.

Because this is a real/possible condition (something you can realistically do now or in general). In Spanish, for real conditions you typically use:

• Si + present indicative, then present/future/imperative in the main clause.
  So Si apago la televisión, puedo… = “If I turn off the TV, I can…”.
  Si apagara/apagase is used for hypothetical/unlikely situations (often paired with podría): Si apagara la televisión, podría concentrarme… (“If I turned off the TV, I could…”).

It’s optional but very common. When the if-clause comes first (Si…), Spanish often uses a comma to separate it from the main clause:

• Si apago la televisión, puedo…
  If you reverse the order, the comma is usually omitted:
• Puedo concentrarme mejor en mis tareas si apago la televisión.

No. Spanish normally drops subject pronouns because the verb ending already shows the subject:

• apago = “I turn off”
  You might add yo only for emphasis or contrast: Si yo la apago (y tú no)…

La is the feminine singular definite article the, and it matches televisión (a feminine noun). You generally cannot omit it here.
You could replace la televisión with a pronoun if it’s already known:

• Si la apago, puedo… = “If I turn it off, I can…”

Grammatical gender in Spanish doesn’t always match real-world gender. Televisión is feminine by convention, so you use:

• la televisión, una televisión, esta televisión, etc.

Puedo (“I can / I’m able to”) fits a real, immediate result: turning off the TV leads to being able to focus better.
Podría (“I could / I might be able to”) would sound more hypothetical or tentative:

• Si apago la televisión, podría concentrarme mejor… = “If I turn off the TV, I could focus better (maybe).”

Concentrarse is a reflexive verb in Spanish: to concentrate (oneself).
So concentrarme = concentrar + me = “to concentrate” (literally “to concentrate myself”).
You’ll often use it reflexively when you mean “to focus” intransitively:

• Me concentro = “I concentrate / I focus.”

Yes, both are correct:

• Puedo concentrarme mejor…
• Me puedo concentrar mejor…
  With two-verb structures (a conjugated verb + an infinitive), object/reflexive pronouns can go before the conjugated verb or attached to the infinitive.

Mejor is the irregular comparative form of bien:

• bien = well
• mejor = better
  Más would need another word (like bien) to make sense:
• puedo concentrarme más = “I can concentrate more” (quantity)
• puedo concentrarme mejor = “I can concentrate better” (quality/effectiveness)

It can mean both, depending on context:

• mis tareas = my tasks (general duties)
• la tarea / las tareas can also mean homework/assignments, especially in school contexts.
  The sentence works for either meaning.

Yes. Apagar is the standard verb for switching off lights/devices:

• apagar la televisión, apagar la luz, apagar el ordenador
  You may also hear apagar la tele (more informal), or sometimes desconectar if you mean unplug/disconnect.

Yes. Spanish allows flexibility. These are both natural:

• Si apago la televisión, puedo concentrarme mejor en mis tareas.
• Puedo concentrarme mejor en mis tareas si apago la televisión.
  The main difference is emphasis: starting with Si… foregrounds the condition.