Learn Hardcore Spanish (Latin America): Cuando me siento cansado, sé que hay que hacer una pausa corta para respirar profundo. - When I feel tired, I know you have to take a short break to breathe deeply.

Estoy cansado = I am tired (a simple state, just describing how you are).
Me siento cansado = I feel tired (focuses more on your subjective feeling, what you perceive in your body/mind).

In many contexts you could swap them, but me siento cansado sounds closer to “I can feel that I’m tired” or “I feel tired,” which fits well with the idea of noticing you need a break.

If a woman is speaking, should it be me siento cansado or me siento cansada?

With sentirse + adjective, the adjective must agree in gender and number with the person speaking:

A man: Me siento cansado.
A woman: Me siento cansada.
Several men / mixed group: Nos sentimos cansados.
Several women: Nos sentimos cansadas.

So a female speaker would say me siento cansada in this sentence.

What’s the difference between sentir and sentirse? Doesn’t sentir also mean “I’m sorry”?

Yes, sentir by itself and sentirse (reflexive) are used differently:

Sentir + noun / clause = to feel (something), to be sorry about (something)
- Siento el frío. – I feel the cold.
- Siento que llegué tarde. – I’m sorry I arrived late.
Sentirse + adjective / adverb / “bien/mal” = to feel (in a certain state)
- Me siento cansado. – I feel tired.
- Me siento bien. – I feel good.

In the sentence, you’re describing your state (tired), so Spanish uses sentirse with a reflexive pronoun: me siento cansado.

Why is there a comma after Cuando me siento cansado?

When a sentence starts with a “cuando…” clause (a dependent clause of time), Spanish normally uses a comma before the main clause:

Cuando me siento cansado, sé que…

If the cuando clause comes second, you usually don’t put a comma:

Sé que hay que hacer una pausa corta cuando me siento cansado.

So the comma is just normal punctuation because the time clause comes first.

What’s the difference between sé and se?

Sé (with accent) is the yo form of saber in the present: yo sé = I know.
- Sé que hay que hacer una pausa. – I know that one should take a break.
Se (without accent) is a pronoun. It can be reflexive (se lava), part of passive/impersonal constructions (se dice), etc.

In the sentence, it clearly means “I know”, so it must be sé with an accent.

What does hay que mean, and how is it different from tengo que?

Hay que + infinitive expresses a general or impersonal obligation: one must / you have to / it’s necessary to…, without saying who exactly must do it.

Hay que hacer una pausa corta. – One should take a short break / It’s necessary to take a short break.

Tengo que + infinitive is personal: tengo = I have to.

Tengo que hacer una pausa corta. – I have to take a short break.

In the sentence, the idea is more general (this is what one should do), so hay que is appropriate.

Why do we need hacer in hay que hacer una pausa corta? Why not just hay que una pausa corta?

Spanish requires an infinitive verb after hay que, and pausa is a noun, not a verb. You need a verb that goes with pausa, and Spanish uses the expression hacer una pausa = to take a break / pause.

So:

hay que hacer una pausa corta = one must take a short break.

You can’t say hay que una pausa because hay que cannot be followed directly by a noun.

Why is it una pausa corta and not una corta pausa?

Both una pausa corta and una corta pausa are grammatically possible, but:

The normal, neutral order is noun + adjective: una pausa corta.
Adjective before the noun often adds a bit of emphasis or a more literary/poetic tone: una corta pausa can sound more stylistic.

In everyday Latin American Spanish, una pausa corta is the most natural and common order.

Why is it respirar profundo and not respirar profundamente?

Standard grammar would suggest the adverb profundamente (deeply), but in everyday Spanish—especially in Latin America—it’s very common to use an adjective like profundo after the verb as an adverbial expression:

Respirar profundo – literally “to breathe deep,” meaning “to take a deep breath / breathe deeply.”

You will also hear:

Respirar profundamente.
Respirar hondo.

All are acceptable; respirar profundo is just a very common colloquial pattern.

Could I say Cuando estoy cansado instead of Cuando me siento cansado?

Yes, that’s correct too, and often interchangeable:

Cuando estoy cansado, sé que… – When I’m tired, I know that…
Cuando me siento cansado, sé que… – When I feel tired, I know that…

Estoy cansado states your condition. Me siento cansado focuses a bit more on your perception of that condition. In many contexts, native speakers use both without a big difference in meaning.

Why do we use para respirar profundo and not por respirar profundo or a respirar profundo?

Para + infinitive expresses purpose: in order to….

…hay que hacer una pausa corta para respirar profundo.
= “…one must take a short break in order to breathe deeply.”

Por + infinitive usually explains a reason or cause, not a goal:

Por respirar tan rápido, me mareé. – Because I was breathing so fast, I got dizzy.

A + infinitive is used in some special patterns (e.g. al llegar = upon arriving), but not for simple purpose. So para respirar profundo is the standard way to express the goal of the pause.

Why is it cuando me siento cansado (indicative) and not cuando me sienta cansado (subjunctive)?

With cuando, Spanish uses:

Indicative for habitual or present-time facts:
- Cuando me siento cansado, hago una pausa. – When(ever) I feel tired, I take a break. (a regular habit)
Subjunctive for future or uncertain events:
- Cuando me sienta cansado, haré una pausa. – When I (eventually) feel tired, I’ll take a break.

In your sentence, you’re talking about a general habit or rule in the present, so me siento (indicative) is correct.