Learn Hardcore Spanish (Spain): No es buena idea escribir un comentario cuando estás enfadado. - It’s not a good idea to write a comment when you are angry.

No es buena idea escribir un comentario…
Sounds a bit more general and idiomatic, like a standard “rule” or piece of advice. Spanish often omits the article with expressions like es buena/mala idea + infinitive.
No es una buena idea escribir un comentario…
Also correct; it can feel slightly more specific or a bit more emphasized, closer to “it’s really not a good idea”.

In everyday speech in Spain, you’ll hear both. The version without una is very common in this fixed expression.

Why is escribir in the infinitive? Why not a conjugated form like escribes?

In No es buena idea escribir un comentario…, the whole phrase escribir un comentario functions as the subject of the sentence:

(El hecho de) escribir un comentario no es buena idea.
= The act of writing a comment is not a good idea.

In Spanish, when you talk about actions in a general way (“doing X is good/bad/etc.”), you normally use the infinitive:

Fumar no es buena idea. – Smoking is not a good idea.
Beber tanto no es buena idea. – Drinking so much is not a good idea.

If you said no es buena idea escribes un comentario, it would be wrong: after es buena/mala idea, you need the infinitive, not a conjugated verb.

You can change the word order and say:

Escribir un comentario cuando estás enfadado no es buena idea.

This just puts the subject at the beginning; the grammar is the same.

Why is there no tú before estás enfadado?

Spanish usually omits subject pronouns (yo, tú, él, etc.) because the verb ending already shows who the subject is:

estás → clearly 2nd person singular (tú).

You only add tú if you need extra emphasis or contrast:

Tú estás enfadado, pero yo estoy tranquilo.
Cuando tú estás enfadado, dices cosas muy duras.

In your sentence, the context is neutral and general, so cuando estás enfadado is the natural choice.

If I’m a woman, should I change enfadado to enfadada?

Yes. Enfadado is an adjective and must agree in gender and number with the person it describes.

Because the understood subject is tú:

A man talking about himself:
No es buena idea escribir un comentario cuando estás enfadado.
A woman talking about herself:
No es buena idea escribir un comentario cuando estás enfadada.

Other possibilities:

Talking to several people (mixed or all male): cuando estáis enfadados
Talking to several women: cuando estáis enfadadas
Formal usted (to a man): cuando está enfadado
Formal usted (to a woman): cuando está enfadada

What’s the difference between enfadado and enojado?

Both mean angry, but usage differs by region:

Spain (Peninsular Spanish)
- enfadado: the most common neutral word for “angry”.
- enojado: understood, but sounds more Latin American or a bit formal/unusual in everyday speech.
- More colloquial in Spain: cabreado (stronger, more slangy).
Latin America (general)
- enojado: very common word for “angry”.
- enfadado: much less common, though still understood.

Since you specified Spanish from Spain, enfadado is the most natural choice here.

Why is it cuando estás enfadado (indicative) and not cuando estés enfadado (subjunctive)?

With cuando, Spanish can use either indicative or subjunctive, depending on meaning.

Indicative (estás): for general situations, habits, facts.
No es buena idea escribir un comentario cuando estás enfadado.
= In general, whenever you are angry, writing a comment is a bad idea.
It sounds like a general rule/principle.
Subjunctive (estés): for future, hypothetical or not-yet-real situations from a specific point of view.
Example:
No voy a escribir nada cuando esté enfadado.
= I’m not going to write anything when I’m (next) angry.

In your sentence, we’re giving general advice, so the present indicative (estás) is the standard and most natural form.

So when would I say cuando estés enfadado?

You use cuando estés enfadado when you’re talking about a future or hypothetical moment, often from the speaker’s point of view. For example:

Cuando estés enfadado, llámame antes de escribir nada.
When you’re (next) angry, call me before you write anything.
No tomes decisiones importantes cuando estés enfadado.
Don’t make important decisions when you’re angry.
(Here the subjunctive makes it feel more like “in any future cases when that happens”.)

You could say:

No es buena idea que escribas un comentario cuando estés enfadado.

Here que escribas forces the subjunctive estés. This structure is also correct but a bit heavier stylistically than your original sentence.

Why un comentario and not just comentarios or no article at all?

Un comentario in this context is like English “a comment” meaning “any (one) comment”, not a specific, known comment.

No es buena idea escribir un comentario cuando estás enfadado.
= It’s not a good idea to write a comment (any comment) when you’re angry.

Other options are possible with small nuance changes:

No es buena idea escribir comentarios cuando estás enfadado.
= It’s not a good idea to write comments when you’re angry.
(More general: writing comments in that state, in general.)
Without article (escribir comentario) is not correct here in standard Spanish; you need un or use the plural comentarios.

Can I say cuando te enfadas instead of cuando estás enfadado?

Yes, but the nuance changes slightly:

cuando estás enfadado
Focuses on the state of being angry.
= when you are (in that state of) angry.
cuando te enfadas (from enfadarse)
Focuses more on the moment/event of getting angry:
= when you get angry / whenever you get angry.

Both are natural in Spanish:

No es buena idea escribir un comentario cuando estás enfadado.
(while you’re angry)
No es buena idea escribir un comentario cuando te enfadas.
(when you get angry; whenever that happens)

In many everyday contexts, speakers might use either one without a big difference in meaning.

Is the word order fixed, or can I change it (for example, Escribir un comentario… no es buena idea)?

You have some flexibility with word order:

Original:

No es buena idea escribir un comentario cuando estás enfadado.

Alternative (perfectly natural):

Escribir un comentario cuando estás enfadado no es buena idea.

Both are common. The second version places more emphasis on the action itself (Escribir un comentario cuando estás enfadado…) as the topic.

What you generally wouldn’t say is something like:

No es idea buena escribir un comentario…

Even though idea buena is not strictly wrong, the fixed, idiomatic phrase is buena idea, so native speakers strongly prefer:

es (una) buena idea, no es (una) buena idea.