President Carter won the year's first test vote on welfare in the House yesterday as a special subcommittee decided 10 to 6 to take the most basic step contemplated in his public-aid plan and extend assistance in cash to all of the nation's poor.

Only certain categories of the poor are aided now.

In its first day of decision, making the special welfare subcommittee also agreed by voice vote to extend this assistance through a new, unified welfare program with national standards of eligibility.

The new program would replace two of the three largest existing programs - Aid to Families with Dependent Children (AFDC), which now helps 21.3 million person, mostly mothers and children, and Supplemental Security Income (SSI), which goes to 5.5 million aged, blind and disabled recipients and their dependants.

Still undecided is whether the third major existing program, food stamps, which goes to SSI and AFDC recipients as well as 2.6 million other persons, also should be replaced.

The subcommittee sidestepped that issue for the time being. It also postponed a decision on how much money the new single welfare program should give people, and what the national eligibility standards should be.

Those are expected to be difficult issues, hammered out only after days and perhaps weeks of debate that could easily last well into next year.

Chairman James T. Corman (D-Calif.) said repeatedly he thought the subcommittee would find it easier to agree first on a broad conceptual framework, then fill in the details.

Many subcommittee members are strongly opposed to key details of Carter's new welfare proposal, such as scrapping food stamps.

One example of the strength of some of that opposition surfaced yesterday in a copy of an alternative welfare plan drafted by advisers to Ways and Means Committee Chairman Al Ullman (D-Ore.), Ullman is a member of the subcommittee, but was not present yesterday.

This Ullman alternative, 3 1/2-page outline lacking in many specifics, calls for keeping the three major welfare programs intact.

It would leave SSI basically untouched except for improvements in administration, and substitute a flat standard deduction for the work expenses, housing expenses and child-care costs that food stamp recipients now can subtract from their incomes before it's decided how much hlep they are entitled to.

For AFDC recipients Ullman's alternative would set a $4,200 minimum income in cash and food stamps for a family of four. Carter's proposal sets the same minimum income, all in cash, for all poor and low-income families of four.

Ullman also proposes a new program. "Temporary Assistance," for two-parent families and individuals between 50 and 65. Recipients of aid under that program would be cut off if they refused to accept minimum-wage jobs, or training or basic education slots.

His outline notes that AFDC recipients would not be required to accept public-service jobs if they can't find regular jobs. Carter's plan calls for the creation of 1.4 million public service jobs for those who are required to work and can't find private employment.

Despite yesterday's tentative votes, any new welfare plan faces a host of obstacles before it can emerge to be voted on by the House.

The subcommittee is not expected to complete work until sometime after Congress returns next year and whatever bill emerges is sure to be controversial.

One congressman who asked not to be identified said yesterday he wasn't sure any welfare bill could be passed by the House next year because it's an election year, and members will be worried about voter opposition to whatever is passed.

"Welfare reform means too many different things to people," he said. "To conservatives it means cutting back federal outlays. To liberals it means increasing benefits. To local officials it means fiscal relief . . ."

Ullman, who called Carter's plan "unworkable" shortly after the President announced it in August, is expected to work out more details of his program, then offer parts or all of it as an alternative if the subcommittee balks at major portions of the Carter plan.

In a series of votes yesterday, mostly along party lines, the subcommittee voted down attempts to exclude from federal assistance ablebedied adult under 40, and those under age 30.

Rep. Guy Vander Jagt (R-Mich.), wholost 15 to 9 on his additional motion to bar individuals and childless couples, said he didn't think the federal government "owed a living" to full-time college students, or able-bodied single adults over 25.

Rep. Barber Conable (R-N.Y.), who proposed barring those under 40, said he wanted to "draw the line at young people."

"I hope we can get that age down to 30 or so." Corman replied. "I suspect that most people under 30will have a job."

The age-40 ban lost 19 to 8, and when Coable immediately moved to ban those below 30, he lost on a voice vote.

Yesterday's votes set the stage for more political maneuvering in the subcommittee.

Rep. Thomas S. Foley (D-Wash.), who as chairman of the House Agriculture Committee, is strongly opposed to scrapping the food stamp program his committee just overhauled, insisted during every step that none of the votes should decide what happens to food stamps.

The subcommittee voted to consider varying any new national eligibility standard according to regional cost-of-living differentials. This was done despite testimony that the data to base them on does not exist for rural areas.