No creo que la fiebre sea un síntoma de este virus.

Questions & Answers about No creo que la fiebre sea un síntoma de este virus.

Why is sea used here instead of es?
The phrase No creo que (I don't think that) expresses doubt or denial, which triggers the subjunctive mood. Since the speaker is doubting that fever is a symptom, we use sea, the present subjunctive of ser, instead of the indicative es. Keep in mind that the affirmative Creo que (I think that) states a firm belief and takes the indicative!
Why do we use a form of ser (sea) for a symptom instead of estar? Isn't a fever temporary?
We use ser here because the sentence is classifying or defining what the fever is in this context: a symptom. When you are identifying something by saying X is a Y, you always use ser, even if the physical condition of having a fever is a temporary state.
Why is it un síntoma rather than una síntoma if the word ends in -a?
Many Spanish words that end in -ma are masculine because they originate from Greek. Síntoma is one of these exceptions, so it takes the masculine article un. Other common examples you have likely seen are el problema, el idioma, and el clima.
Why does the sentence use este virus instead of esto virus?
Este is the masculine singular adjective for this, used when it comes directly before a noun. Esto is a neuter pronoun used only when you are saying this without naming a specific object, such as when referring to an abstract idea or an unknown thing. Since virus is a masculine singular noun, it requires este.

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