preventi

Not pre- plus venti, and no wind is involved. This is the false seam of the band, and it is a good one, because both halves look like Esperanto. There is a root vent- in vento, the wind. There is a pre- in words like prezidi that a hopeful learner might read as a prefix. Put them together and you get a picture: blowing something away before it arrives. Charming, and wrong at both ends.

Esperanto has no pre- prefix. The word for before as a prefix is antaŭ, which is why antaŭvidi is to foresee and antaŭdiri is to predict. And the vent in preventi is not the wind: it is Latin venire, to come, the same root sitting inside konveni, inviti and English intervene. Praevenire meant to come before, in the sense of getting there first and thereby settling the matter.

This is the inspiri class again: a borrowed root whose Latin decomposition is real history and dead grammar. Prevent- is one root under Rule 15 and does not come apart.

Now the part that matters in use, because the gloss "to prevent" will get you into trouble. Esperanto keeps three ideas separate where English uses one word:

• Malhelpi - to hinder, to stop something happening, to get in the way of it. This is the ordinary everyday prevent. La pluvo malhelpis nin - the rain prevented us.
• Eviti - to avoid, to steer round something.
• Preventi - to take measures in advance so that a thing never arises. It is the technical and medical word: preventa medicino is preventive medicine, and preventi malsanon is what vaccines do.

If you are stopping something already under way, you want malhelpi. If you are stopping it before it begins, preventi is exactly right and nothing else quite covers it.

Derived forms. Prevento is prevention, preventa is preventive, and preventilo is a preventive measure or device. Note the transitivity: you preventas the illness, not the patient.