Can anyone translate this Latin "oracular response"?

I'm reading about an ancient temple at which an oracle of the goddess Fortuna was housed. If a visitor came seeking divine guidance, the priest or priestess of the temple would reach into an urn containing small bronze tablets. Each tablet was pre-inscribed with a response, and the one that was randomly selected (under the guidance of Fortuna, presumably) would be regarded as containing the goddess' answer to the question. So needless to say, each of these inscriptions was written in a vague, mysterious sounding way. A bit too mysterious for me to have much success in deciphering them!

Many of them are written in a similar pattern, so if someone could translate one for me I probably can manage to crack at least some of the others. Here is one, with punctuation supplied by the author of the book I am reading:

"De incerto certa ne fiant; si sapis, caveas."

I get the part after the semicolon. But I am thown by the two adjectives, one singular and one plural, referring to uncertainty and certainty. And I am not sure what the plural subject of fiant would be.