Hoy me siento bien, aunque estoy cansado.

Spanish has two related verbs:

• sentir = to feel (something), to sense, to regret
  
  • Siento frío. = I feel cold.
  • Siento mucho lo que pasó. = I’m very sorry about what happened.
• sentirse (reflexive) = to feel (a certain way, a state or condition)
  
  • Me siento bien. = I feel well.
  • Me siento triste. = I feel sad.

In your sentence, you’re describing how you feel (your state), not what you feel as an object. That’s why you must use the reflexive form me siento, not plain siento.

Both can translate as “I feel good / I’m fine”, but there’s a nuance:

• Me siento bien focuses more on your internal feeling at that moment (physically or emotionally).
• Estoy bien is more general: “I’m fine / I’m OK”, a broader state, not necessarily emphasizing the sensation.

In many everyday situations you can use either and be understood, but me siento bien explicitly highlights the subjective feeling.

In Spanish:

• bien is an adverb (how you feel, how something is done).
• bueno is an adjective (describes a noun).

With verbs like estar and sentirse, when you’re describing how you feel (an adverbial idea), you normally use bien:

• Me siento bien. = I feel well.
• Estoy bien. = I’m fine.

You would use bueno to describe a thing or person:

• Un día bueno. = a good day.
• Un libro bueno. = a good book.
• Él es bueno. = He is a good person.

So me siento bueno would sound strange here; me siento bien is the natural choice.

Aunque can take either indicative or subjunctive:

• Indicative (aunque estoy cansado) is used when the speaker sees the situation as a real, known fact.

  • Hoy me siento bien, aunque estoy cansado.
    = “I feel good today, even though (and yes, it’s true that) I’m tired.”

• Subjunctive (aunque esté cansado) is used for hypothetical, unknown, or less “real” situations.

  • Me sentiré bien, aunque esté cansado.
    = “I will feel good, even if I’m (might be) tired.”

In your sentence, the speaker is clearly actually tired, so the indicative estoy is correct.

The verb choice ser vs. estar matters:

• estar is used for temporary states and conditions (how you feel now).
• ser is used for permanent or defining characteristics (or roles, origin, time, etc.).

Being tired is usually a temporary state, so you say:

• Estoy cansado. = I am tired (right now, these days).

Soy cansado is almost never used and would sound unnatural in most contexts. It might only appear in a special, figurative sense like “I’m a tiring person,” but even then Spanish usually uses a different phrasing.

Yes. Cansado is an adjective and must agree with the subject:

• Male speaker, singular: Estoy cansado.
• Female speaker, singular: Estoy cansada.
• Mixed or all-male group: Estamos cansados.
• All-female group: Estamos cansadas.

In Hoy me siento bien, aunque estoy cansado, we assume the speaker is male. A female speaker would say …aunque estoy cansada.

Yes, that’s grammatically correct and natural. The meaning is almost the same, but the emphasis shifts:

• Hoy me siento bien, aunque estoy cansado.
  Focus: You feel good; being tired is a secondary contrast.

• Hoy estoy cansado, aunque me siento bien.
  Focus: You’re tired; surprisingly, you still feel good.

Word order in Spanish can subtly change what is highlighted, just like in English.

Yes. Common possibilities include:

• Hoy me siento bien, aunque estoy cansado.
• Me siento bien hoy, aunque estoy cansado.
• Me siento bien, aunque hoy estoy cansado.

All are acceptable. The position of hoy slightly changes what you make more salient:

• At the beginning: emphasizes today as the frame for everything.
• Near me siento: focuses on feeling good today.
• Near estoy cansado: highlights being tired today in particular.

Spanish is a “pro‑drop” language: the verb ending shows the subject, so the subject pronoun is often omitted.

• Me siento already tells you it’s I feel, because -o is the yo ending.
• Estoy also has the yo ending from estar.

You can say Hoy yo me siento bien, aunque yo estoy cansado, but it sounds heavy and is only used for special emphasis (for example, contrasting yo with someone else). Normally, you just drop yo.

You can say Me estoy sintiendo bien, but:

• Me siento bien is the most natural, default way in Spanish to say “I feel good” right now.
• Me estoy sintiendo bien sounds more like a process: “I’m starting to feel good / I’m feeling good (these days).”

So it’s not wrong, but for a simple, general statement of how you feel now, Me siento bien is preferred.

In this sentence, it’s standard and recommended to use the comma:

• Hoy me siento bien, aunque estoy cansado.

You’re joining two related clauses, and the comma often appears before conjunctions like aunque, pero, sin embargo when they contrast ideas. You might see it written without a comma in informal contexts, but the comma is the more correct, clear option.

Bien is intentionally broad:

• It can be physical: “I feel physically OK / not sick.”
• It can be emotional: “I feel good emotionally / in a good mood.”
• Often it means a mix of “OK overall”, just like English “I feel good / I’m fine.”

Context would clarify which aspect is more important. On its own, Me siento bien or Hoy me siento bien usually just means “I’m doing/feeling fine today.”