The Securities and Exchange Commission has proposed new rules that require fuller and earlier disclosure of tender offer plans by a company that intends to make a bid for another.

The new rules also seek to curb trading by insiders who buy or sell a company's stock after learning privately that a tender offer is in the works for that company.

The proposals, which were discussed Wednesday at an open meeting of the commission, will be published for public comment in the next several days.

In a separate development, the commission adopted rules prohibiting certain accounting practices. The rules, passed over the dissent of Commissioner Roberta Karmel, further define bookkeeping practices made illegal under the Foreign Corrupt Practices Act.

Under existing SEC rules, a bidder that intends to make a tender offer for a company's stock does not have to reveal his intentions for 10 days after he acquires 5 percent or more of the target's stock.

The new rules, if adopted, would force the bidder to reveal his intentions as soon as he begins acquiring shares of the target company.

Tender offers would remain open for 30 business days under the proposal. An earlier proposal. made by the SEC in 1976 and never adopted, put a minimum of only 15 business days for an offer.

The rules would also make it possible for stockholders who have tendered their shares, to get the higher price for their shares if the bidder subsequently increases his offer.

A section of the proposal would inhibit insider leaks on tender offers that put the seller, who doesn't have the same information, at a disadvantage.

For example, if an executive tells a friend of an impending tender offer and the friend buys stock on that in side knowledge, the friend must reveal this or he will be violating the law.

Also, if the bidder realizes someone is acting on inside information, this must be made public immediately -- even if the formal tender offer has not been announced.

Chairman Harold Williams, in his remarks prior to the discussion of the proposed tender rules, said the rules would help forestall the mysterious run-up in the stock price of target companies which has become common.

The Feb. 5 issue of Forbes magazine lists 40 of the biggest corporate takeovers of 1978 and notes: "Many of these stocks moved up substantially in the month prior to the announcement of the offer."

The accounting rules adopted Wednesday make it illegal for anyone to "directly or indirectly, falsify or cause to be falsified, any book, record or account..." that is subject to federal securities laws.

The rules also prohibit corporate directors or officers from deceiving an accountant in connection with the preparation of an audit for official corporate document.