Learn Hardcore Spanish (Latin America): Cuando me siento triste, veo una estrella fugaz.

The reflexive form sentirse means to feel (emotion) internally. Using me siento triste indicates that the sadness is something you experience yourself. Saying siento triste without me would literally mean I sense sadness (like detecting it externally), which sounds odd. In Spanish, you always use the reflexive pronoun for personal feelings: me siento feliz, me siento cansado, etc.

Could I use the verb estar instead and say Cuando estoy triste, veo una estrella fugaz?

Yes. Cuando estoy triste... is perfectly correct. Estar triste is actually more colloquial and very common for describing emotional states. Sentirse triste simply emphasizes the act of feeling sadness. Both options convey the same basic idea.

Why is the sentence in the present indicative (me siento, veo) and not the future or the subjunctive?

Here Cuando introduces a habitual or general action—whenever you feel sad, you see a shooting star. For repeated or general facts, Spanish uses the present indicative in both clauses. You’d use the subjunctive only for specific future events:
• Habitual/general: Cuando me siento triste, veo una estrella fugaz.
• Future/specific: Cuando me sienta triste, veré una estrella fugaz (if/when it happens then).

What does una estrella fugaz literally mean, and how does fugaz translate?

Una estrella fugaz literally means a fleeting star. In everyday English it’s called a shooting star. The adjective fugaz means fleeting, short-lived, or transient, emphasizing how quickly the star appears and then disappears.

Why is there a comma after triste?

When a subordinate cuando-clause comes before the main clause, it’s standard in Spanish to use a comma to signal the pause:
Cuando me siento triste, veo una estrella fugaz.
If you reverse the order—Veo una estrella fugaz cuando me siento triste—you generally omit the comma.

Can I say cuando me pongo triste instead of me siento triste?

Yes. Ponerse triste means to become sad, focusing on the onset of the emotion.
• Cuando me pongo triste, veo una estrella fugaz = When I start feeling sad, I see a shooting star.
Using me siento triste simply states when I feel sad, without emphasizing the change.

Is it possible to make the sentence plural, like Cuando me siento triste, veo estrellas fugaces?

Absolutely. Changing to estrellas fugaces (plural) suggests you see more than one shooting star whenever you’re sad.
• Singular una estrella implies one per occasion.
• Plural estrellas implies multiple sightings. Both are grammatically correct.