Learn Spanish (Latin America): Si llueve mucho, habrá gusanos en la acera. - If it rains a lot, there will be worms on the sidewalk.

This is the golden rule of impersonal haber (when it means "there is/are" or "there will be"): it is always singular! Just as you use hay for both "there is one worm" and "there are many worms," you must use the singular future form habrá for plural objects. While you might occasionally hear native speakers mistakenly say habrán, it is considered grammatically incorrect.

Is gusanos the subject of the second half of the sentence?

No, and this is exactly why the verb doesn't become plural! In impersonal sentences with haber, there is no subject. The thing that exists (gusanos) is actually the direct object. Since there's no plural subject for the verb to agree with, the verb stays in its default singular form.

Why do we use the present tense llueve when the sentence is talking about a future possibility?

When making "if/then" statements about likely future events (the First Conditional), Spanish requires the present indicative tense immediately after si (if). You then pair it with the future tense in the result clause (habrá). Saying si lloverá would be a direct translation from English that doesn't work in Spanish.

I've learned acera for "sidewalk", but is it the most common word for it in Latin America?