Sometimes Italian uses a non that doesn't make the sentence negative and isn't translated into English. This is called the "pleonastic" (redundant) non.
It adds emphasis and stylistic flair in subjunctive complement clauses following verbs of fearing, hindering, or doubting.
Fearing
With verbs like temere (to fear), Italian speakers often insert a pleonastic non into the che clause.
| I fear the pests might invade the vegetable garden. | Temo che i parassiti non invadano l'orto. |
Notice that the English translation is "might invade," not "might not invade." The non does not negate the verb; it just adds a subtle sense of apprehension.
| We feared the pests might gnaw the leaves on the balcony. | Temevamo che i parassiti non rosicchiassero le foglie sul balcone. |
Preventing and Hindering
You will also frequently see the pleonastic non after verbs that express stopping an action, such as evitare (to avoid/prevent) and impedire (to impede/prevent). The non reinforces the negative outcome you are trying to avoid.
| I spray the repellent to prevent the snails from entering. | Spruzzo il repellente per evitare che le lumache non entrino. |
| This wall prevents the worms from entering the garden. | Questo muro impedisce che i vermi non entrino nel giardino. |
It works exactly the same way with verbs like ostacolare (to hinder).
| I use this precaution to prevent the parasite from hindering the growth. | Uso questa precauzione per evitare che il parassita non ostacoli la crescita. |
Expressing Doubt
Finally, verbs of doubting like dubitare can also trigger the pleonastic non. Just as with fearing and hindering, the non does not flip the meaning of the clause; it simply colors the doubt.
| I doubt that the repellent works. | Dubito che il repellente non funzioni. |