It's tough to write a constitution that will endure

To most United States citizens, the word ''constitution'' means an 18 th-century document, written on parchment with quill pens by men wearing knee britches.

But in much of the rest of the world, constitutions are modern inventions, like word processors and cable television. Over half the constitutions now in effect around the globe were written after 1970.

The Dutch, for instance, adopted a new one last February. Canada's constitution dates from 1982. Liberia is aiming to produce a new national charter in time for a scheduled 1985 return to democracy.

''This tremendous explosion in constitution writing shows two things: They are taken very seriously, and it is very hard to write them so they will last,'' says Robert Goldwin, head of the American Enterprise Institute's Constitution Project. Last week in Washington, AEI gathered together constitution authors from 20 nations to trade tips about how and why these documents are drawn up. Such shop talk is valuable, says Dr. Goldwin, because constitution writing is very much a thriving industry.

Only six nations don't have constitutions. The United Kingdom, New Zealand, and Israel all have series of fundamental laws that substitute as national charters. The Koran is the official constitution for Saudi Arabia and Oman. Libya is ruled by Col. Muammar Qaddafi's decrees.

The US Constitution, adopted in 1789, is easily the world's oldest. Only four other substantially unaltered constitutions were written before 1900: Norway ( 1814), Argentina (1853), Luxembourg (1868), and Colombia (1886). In fact, only 15 of the world's approximately 160 constitutions existed prior to World War II.

Part of the modern-day surge in constitutions, of course, is due simply to a surge in new nations - especially in sub-Saharan Africa - over the last 20 years.

But much of it stems from the fact that, outside the US, it's normal for constitutions to break down or be thrown out.

''In some other countries, they just do not think of writing for the ages,'' says Goldwin.

At least one-third of the constitutions written since 1970 were simply replacements for old models made obsolete by time. And governments that seize power in a coup or revolution almost always junk the old regime's charter and draft a new one.

Liberia's original constitution was written in 1847, for instance. But it was voided by head of state Samuel Doe when he seized power in a 1980 coup. A new constitution is scheduled to be ready by 1985, when Mr. Doe has said he will hold elections.

The new charter, says Amos Sawyer, head of Liberia's constitution drafting committee, will feature a weaker presidency, and will encourage development of political parties that cut across tribal boundaries.

''A constitution is important, not only for symbolic reasons, but to provide a kind of structure that will help'' diverse people become one nation, says Dr. Sawyer.

The US Constitution, despite its venerable status, is not a direct influence on the constitutions of today, say scholars.

Other national charters have become more popular role models: West Germany's constitution is much admired for its section on social rights. The constitutions of Mexico (1917), Ireland (1937), and India (1949) have also become classics, says Albert Blaustein, a Rutgers University law professor.

Mr. Blaustein himself is something of an influence in the field, since he also heads a firm billed as ''an international consortium of constitutional consultants.''

He says a good constitution is ''homegrown,'' reflecting the political needs of a people. His role as a constitution-writer-for-hire, he says, is limited to technical advice.

''When they want to know what you do to put in executive veto power, I tell them. That kind of thing,'' he says.

As a consultant, Blaustein even has suggestions as to what the US founding fathers could have done differently. For one thing, he says, it's ''foolish'' to throw presidential elections into the House if neither candidate has an electoral majority. He favors simply having a follow-up election.

''Another thing I would have done was put in Sunday voting. Why vote on Tuesday? It's ridiculous.''

The US Constitution is admittedly not perfect. But as much as any document of its kind, it has fulfilled the function of truly ''constituting'' a nation, says Goldwin.

''There are a lot of dishonest constitutions in the world,'' he says, ''whose words bear no relation to the way the country is governed. But an honest one is consulted every day. Wherever there is an honest constitution, it has a supremacy.''