Learn Portuguese (Portugal): Não acho que ela esteja a estragar o penteado com tanta laca. - I don't think she's ruining her hairstyle with so much hairspray.

The phrase esteja a estragar is the Continuous Subjunctive, made with the Present Subjunctive of estar plus a and the infinitive. We use it to emphasize that the action is happening right now. If we used the simple Present Subjunctive (estrague), it would sound more like a general statement: I do not think she ruins her hairstyle.

What exactly triggers the subjunctive esteja in this sentence?

The subjunctive is triggered by Não acho que (I do not think that). While the affirmative Acho que expresses a concrete opinion and takes the indicative, making it negative introduces doubt, which requires the subjunctive in Portuguese.

Why does the Portuguese sentence say o penteado (the hairstyle) instead of o seu penteado (her hairstyle)?

In Portuguese, it is standard to use the definite article instead of a possessive pronoun when the context makes it obvious who the object belongs to. This is the default rule for body parts, clothing, and personal grooming aspects like o penteado.

Why do we use tanta instead of tão for "so much"?