YOU DON'T have to be a legal expert to know the difference between a married woman and a marriageable one. The first adjective describes a condition that exists and can be determined with certainty; the second indicates only a potential. The same distinction applies to the words "classified" and "classifiable" -- with a vengeance. It would be foolish to penalize a man for flirting with a woman because he should have known that some day she would be married to someone else. But it would be downright outrageous to punish a government employee for revealing unclassified material on the grounds that he should have known it would be classified some day, as the Reagan administration threatens to do.

Standard Form 189, which must be signed by government workers with security clearances, has required a pledge not to disclose "classified or classifiable" material. Failure to sign means a loss of clearance, which in many cases puts a job in jeopardy. Civilian workers for defense contractors refused to sign a similar form, and it was rewritten for them to remove the vague and open-ended word "classifiable." But those who work for Uncle Sam have not been so successful.

Instead, they are given assurances by the Information Security Oversight Office that the word has a restricted, easily understood meaning. This week, for example, the ISOO sought to counter congressional protests by adding this dizzying sentence to the regulations published in the Federal Register: "The term "classifiable" does not include any information that is not otherwise required by statute or Executive order to be protected from unauthorized disclosure in the interest of national security." Does that clarify for every clerk in the government which unmarked documents he can read aloud on a street corner and which he can lose his job for revealing?

Explanations, clarifications and new definitions won't do. There is only one way to deal with the uncertainty and potential for abuse inherent in the language. Remove the word "classifiable" from the form. Material is protected if it is clearly marked as classified. So are copies, photographs or notes taken from the material. But until a determination is made that a document must be protected for national security reasons and until that designation is clearly attached, government papers are public property and available for public discussio