Learn Hardcore Spanish (Latin America): Cuando estés muy cansado, no deberías seguir leyendo durante horas y horas. - When you are very tired, you shouldn’t keep reading for hours and hours.

In Spanish, when cuando introduces a future or hypothetical situation, it normally triggers the present subjunctive, not the present indicative.

Cuando estés muy cansado… ≈ When you are very tired (in that future/possible moment)…
→ Future / not a specific, known moment yet → subjunctive (estés)
Cuando estás muy cansado, siempre te duermes en el sofá.
→ General habit / repeated, factual situation → indicative (estás)

So in the original sentence we’re talking about whenever / when you (find yourself) very tired in the future, giving advice about that situation, so estés (subjunctive) is used.

Could I say Cuando estás muy cansado, no deberías seguir leyendo…? What would change?

You can say it, but the nuance changes:

Cuando estés muy cansado…
→ Sounds like conditional advice for future or possible situations that may arise. More like: Whenever you (happen to) be very tired, you shouldn’t keep reading…
Cuando estás muy cansado…
→ Sounds more like a description of a usual pattern or habit: When you’re very tired (as a rule), you shouldn’t keep reading…

Both are grammatically correct, but for general advice about what to do in that future case, estés (subjunctive) is more natural.

Why is it present subjunctive (estés) if we’re talking about the future?

In Spanish, after time words like cuando, en cuanto, hasta que, después de que, you don’t normally use a future tense in the subordinate clause.

Instead of:

✗ Cuando estarás cansado… (incorrect)

You say:

Cuando estés cansado… (present subjunctive)

The future meaning comes from the context and from the main verb (no deberías seguir…) and from the nature of the clause (a condition that hasn’t happened yet). So:

English: When you are very tired, you shouldn’t keep reading… (future idea in context)
Spanish: Cuando estés muy cansado, no deberías seguir leyendo… (present subjunctive used for that “future when” idea).

What is the difference between no deberías and no debes here?

Both are possible, but the tone changes:

No deberías seguir leyendo…
→ Soft advice, recommendation, suggestion. Like you really shouldn’t / it’s better if you don’t.
No debes seguir leyendo…
→ Stronger obligation or rule. Like you must not / you’re not allowed to.

In everyday speech, no deberías is more polite and less bossy. That’s why it fits well with advice about what someone should do when they’re very tired.

Why is it the conditional deberías and not debes if it’s advice, not hypothetical?

Spanish often uses the conditional to soften advice or suggestions, even if the situation is very real, not hypothetical:

Deberías estudiar más. = You should study more. (gentle advice)
Debes estudiar más. = You must study more. (stricter, more authoritative)

In the sentence:

no deberías seguir leyendo…

the conditional deberías makes it sound like a friendly recommendation, not a direct order.

Why is it seguir leyendo and not something like continuar a leer or just leer?

Seguir + gerund is the most natural way in Spanish to say “to keep / continue doing something”:

seguir leyendo = to keep reading
seguir trabajando = to keep working
seguir hablando = to keep talking

Using continuar a leer is not natural; with continuar, you’d also typically use the gerund:

continuar leyendo (also fine, but a bit more formal/neutral)

Just no deberías leer durante horas y horas would mean you shouldn’t read for hours and hours at all, while no deberías seguir leyendo emphasizes you are already reading and you should not keep doing it.

What exactly does leyendo mean here? Is it like the English “reading” as a noun?

Leyendo is the gerund of leer (to read) and here it forms a periphrasis (verb phrase) with seguir:

seguir + leyendo = to keep on reading

It is not a noun in Spanish. In English you can say “I like reading” where “reading” acts like a noun; in Spanish you’d normally use the infinitive:

Me gusta leer. (not ✗ me gusta leyendo in this sense)

In this sentence, leyendo is part of the verb phrase describing an ongoing action, not a noun.

Why is it cansado and not cansada? What if I’m talking to a woman?

Cansado agrees in gender (and number) with the subject:

Speaking to or about a man: (tú) estás cansado
Speaking to or about a woman: (tú) estás cansada

In the sentence, the form cansado suggests we’re addressing a male person (or using a generic masculine). If talking to a woman, you’d say:

Cuando estés muy cansada, no deberías seguir leyendo durante horas y horas.

Why isn’t the subject tú written? Could we say Cuando tú estés muy cansado?

Spanish normally drops subject pronouns when the verb ending already shows the person:

(Tú) estás
(Tú) estés

So Cuando estés muy cansado… is completely natural and actually more common.

You can say Cuando tú estés muy cansado…, but adding tú usually:

Emphasizes the subject (contrast: when you are tired, not when others are), or
Sounds slightly heavier in everyday speech.

In neutral advice with no special emphasis, the version without tú is preferred.

What does durante horas y horas add? Is it necessary, and can I say por horas y horas?

Durante horas y horas literally means for hours and hours and adds the idea of a very long time, in an exaggerated way.

It’s not strictly necessary; you could say:
…no deberías seguir leyendo. (no extra emphasis on duration)
Horas y horas is a repetition for emphasis, common in Spanish.

You can say por horas y horas, and people will understand, but:

durante horas y horas is more standard and natural in most contexts.
por horas is also common (especially in some regions), but durante tends to sound a bit more neutral/formal.

Why is there a comma after Cuando estés muy cansado? Is it required?

In Spanish, when a subordinate clause (like a cuando-clause) comes before the main clause, you normally separate them with a comma:

Cuando estés muy cansado, no deberías seguir leyendo…

If you invert the order, you usually remove the comma:

No deberías seguir leyendo durante horas y horas cuando estés muy cansado.

So in the original order, the comma is standard and expected.

Could we use tan cansado instead of muy cansado? Would the meaning change?

[ANSWERER]
Yes, both are possible but they feel slightly different:

muy cansado = very tired (strong degree, but neutral)
tan cansado = so tired (often feels like it leads to a consequence)

For example:

Cuando estés muy cansado, no deberías seguir leyendo…
→ General advice: whenever you are very tired, don’t keep reading.
Cuando estés tan cansado, no deberías seguir leyendo…
→ Suggests so tired (that continuing really doesn’t make sense / is harmful). Often tan appears with a continuation:
Cuando estés tan cansado que ya no puedas concentrarte…

In isolation, muy cansado is the more neutral, common choice for simple advice.