The nation's monthly foreign trade deficit narrowed slightly last month, resuming the gradual improvement that was interrupted in September and October, the government reported yesterday.

The excess of imports over exports fell to $1.95 billion in November, down from $2.13 billion the previous month, the Commerce Department announced.

However, the cumulative deficit for the first 11 months of this year hit a record $26.7 billion, pointing to a redink figure of $28 billion or more for 1978 as a whole. Last year's deficit was $26.5 billion.

The November decline in the trade deficit marked good news for the Carter administration, which has been asserting for serveral months that the nation's trade posture would improve.

Almost immediately after the trade figures were released, the dollar's value shot upward on the major world currency markets. Analysts had blamed the large trade deficit earlier for the dooar's decline.

Carter administration officials were visibly pleased by the latest development. Commerce Secretary Juanita Kreps issued a statement saying the decline "clearly indicates that the trade fijures are improving."

At the same time, the department announced that its index of leading economic indicators -- whose behavior is supposed to foreshadow changes in the economy's performance -- fell 0.6 percent in its first decline since July.

However, analysts noted that the major factor behind the new dip was a slowdown in the growth of the money supply, which most economists have been viewing as good. Observers attached little importance to yesterday's figure.

Although the narrowing in the November trade deficit was relatively modest, the figures actually marked a significant improvement in the trade posture -- beyond what the overall statistics suggested.

For the thing, the gains came despite another increase in the value of goreign oil imports, which rose $81 million over the month after falling somewhat in October. The rise in November amounted to 2.3 percent.

For another, the figures for the first time months showed a significant surplus in manufactured goods, indicating the United States finally is regaining some ground in this key sector of foreign trade.

Exports of manufactured goods exceeded imports in November by $360 million, with a further decline in automobile imports, except for cars from Canadn. Farm exports declined, but their levels fluctuate sharply.

Overall exports jumped a sharp $251 million, or 1.9 percent, in November, after a 3.1 percent decline the previous month. At the same time, imports rose 0.5 percent, following an 0.1 percent increase in October.

Meanwhile, the Treasury announced yesterday it has begun an "anti-dumting" investigation involving the sale of carbon steel plate by five European steel producers -- a possible first step to imposition of special quotas.

The move followed by one day complaints by David Roderick, president of United States Steel Corp., that the administration was hot adequately enforcing its new "trigger-price" plan for preventing cut-rate imports.

The action came on a complaint filed by the Lukens Steel Co. of Coatesville, Pa., charging that American firms were bing "substantially undersold" because the foreign producers were selling below fair value.

Separately, the Commerce Department also announced the U.S. trade deficit with Japan declined in Movember to $674 million, from $838 million in October. The deficit with Japan has been a major concern to policymakers here.

Kreps also took pains in her statement yesterday to brush aside any suggestions that U.S. buyers may be stepping up their purchases of foreign oil to get in under the the wire before next month's 1 percent oil-price increase.

The commerce secretary noted in her remarks that oil imports for the first 11 months of 1978 have been running some 9 percent below last year's level. "There is no evidence yet," she