Mahroos Meabad, a 62-year-old fruit wholesaler, was giving his nephew a lesson in Egyptian patience the other night as the two sat sipping tea at an outdoor cafe overlooking the lights of Cairo.

"At least there is plenty of work now," he told the younger man, who was complaining that peace with Israel had yet to bring Egyptians its promised prosperity. "These things take time. The work we are doing now will show results in time."

Meabad's nephew, 37, was still not convinced that President Anwar Sadat's treaty is bringing a better life to the Egyptian people fast enough. But he was certain, along with what appears to be a vast majority of the country's 41 million inhabitants, that sooner or later the new era of relations with the Jewish state will usher in a new era of prosperity for Egypt.

By building that certainty, Sadat and his government have won broad support among Egyptians for the end of hostilities with Israel and the determination to persevere in peaceful relations despite estrangement from the rest of the Arab world.

At the same time, however, they have raised the danger that if the ponderous state-oriented Egyptian economy does not visibly improve the lot of Egypt's poor in the coming few years, the certainty could turn to doubt and from doubt to resentment. Despite a massive infusion of U.S. economic aid -- estimated at $1.1 billion this year -- a look around Cairo shows how far Egypt has to go to keep the certainty alive:

Three persons were killed last month when their mud-brick house collapsed. The government estimates 12,000 houses collapse in Cairo every year, putting about 60,000 Egyptians out of their homes.

Egyptair, the national airline, recently ran an ad in Cairo newspapers for stewards and stewardesses. Starting pay was set at $42 a month, about what a housemaid makes.

The wife of a Cairo laborer was grilling mutton last month in one of the city's poor quarters. "Do you smell that smoke going out the window?" her husband said, glowing with pride. "All the neighbors are going to know we're having meat tonight."

The government, reacting to a shortage, recently announced that imports of frozen meat was increased from 1,700 tons a month to 4,000. Then Supply and Commerce Minister Nassif Tahoon reassured the public that subsidized government stores "will sell meat until the end of the line." Several days later hundreds of black-robed women were seen lined up outside one of the stores near a truckload of frozen meat parked in the street.

Despite such hardships, conversations in the streets of Cairo still show a remarkable degree of optimism that, with the war over, the days of easier living are approaching. Sadat's aides have given clear directives to the government-guilded press to cultivate and sustain this impression, according to Cairo journalists.

For now, only narrow groups on Egypt's far left and far right can be seen trying to swim against the current. Neither enjoys wide backing but both are tightly controlled by the government and periodic arrests of their sympathizers are reported.

On the left, the main group is the Progressive Unionist Rally, run by educated leftist who generally remain loyal to the pan-Arab principles of the late president Gamal Abdul Nasser and thus oppose Sadat's separate peace with Israel. Leader Khaled Mohiddin claims a membership of 30,000 across the country, but the group has been unable to get anyone elected to parliament.

A second group -- to the left of Sadat's National Democratic Party but regarded as a moderate "official opposition" -- is the Socialist Labor Party.

With only about 30 seats in 392-seat parliament, it has avoided taking strong official positions against the peace treaty. But some members have voiced their own opposition and the party organ, The People, has sharply criticized Sadat on other issues.

On the right are Islamic Fundamentalist groups that oppose the Jewish state out of the Moslem solidarity and religious fanaticism, sometimes including traditional anti-Semitism. The most prominent of these is the Moslem Brotherhood, which is tolerated in Egypt but tightly controlled and strictly supervised.

Its weekly organ, The Call, observed the arrival of Israel's first ambassador here last month by complaining:

"Israel is going to drown Egypt in pornographic magazines, encourage moral decay, open night clubs and build hotels where ignoble vices such as striptease, gambling and dancing will flourish."

The strength of Islamic extremist groups is difficult to measure, informed analysts here say, because it lies more in a potential for rousing the masses of Cairo than in formal membership. While the Moslem Brotherhood operates openly, several other groups including the Islamic Association and Jihad, function in the shadows.

A source who has studied Egypt's fundamentalist movements said their appeal is increased by eagerness to do practical favors in the manner of a Chicago ward politician. The Islamic Association is good at organizing bus tours for Cairo University students, for example, and a popular fundamentalist, Sheik Kishk, regularly hand out money to housewives who line up at his mosque to recount their woes.

Kamel Nafadi, a cafe owner in Cairo's low-income Qalaat Kafsh quarter, said students from the Islamic Association often paste posters on neighborhood walls and urge his customers to go the mosque and pray instead of drinking coffee. About 2,000 of them held a rally in the celebrated Al Azhar mosque March 7 to denounce nomalization and warn against Jewish penetration into Egypt.

A Jihad hideout with a stock of weapons and explosives, discovered recently in Alexandria, was said by the government to be part of a plot encouraged to Egypt's enemies abroad -- presumably Libya. Authorities have reported more than 120 arrests in the last several months connected with that extremist organization.

After witnessing Iran's Islamic upheaval and the mosque takeover by religious extremists in Saudi Arabia Sadat's government appears determined to prevent such fundamentalist sentiments from spreading here and being exploited against the rule.

About three thousand Islamic fundamentalists led by students from the university in Asiut, south of Cairo, demonstrated Friday night against the former shah's presence and normalization of relations with Israel. The demonstration prompted police to intervene with tear gas. Islamic students at Cairo Univeristy issued a communique saying one of their number had been killed by police during the Asiut disorders, but interior Ministry officials denied this. The Cairo students staged their own demonstration -- this one peaceful -- on their campus last Wednesday.

Sadat himself sought cooperation with Sheik Hassan Banni of the Moslem Brotherhood during his days as part of the military officers' plot against the monarchy, but now it is he who is in power.

Protection of Islam is joined to protection of the government in the so-called "law of shame," controversial draft legislation being prepared by Sadat's party to codify ways of halting challenges to his vision of Egypt. In another reflection of concern, the semiofficial Cairo newspaper Al Ahram recently published a series of articles warning against the dangers of religious fanaticism.

The most experienced observers here say, however, the Egypt's Islamic rightist could present a serious challenge to Sadat's peace with Israel and friendship with the West only if they rode a wave of economic frustration into mass street protests.

Mindful of this, and of the violent demonstrations here in January 1977, the Sadat government has allowed a mammoth deficit to build up in order to maintain subsidies that make day-to-day living possible for many of Egypt's millions. It was a decision to reduce some of the subsidies -- later retracted -- that sent protesting mobs into the Cairo streets in 1977.

Western economists insist a major overhaul of the subsidy system is needed if Sadat wants to reduce a budget deficit of $2.5 billion, about one-fifth the Egyptian gross national product. The International Monetary Fund has made such a reduction a condition for new credits in negotiation now under way.

Prices were raised for gasoline, cigarettes and several other subsidized products during 1979. But Sadat's ministers still are reluctant to trim broad, expensive subsidies such as those that keep bread prices at one cent a plate-sized flat loaf and frozen meat at 50 cents a pound in government stores.

Despite the subsidies -- and largely because of the deficits they cause -- inflation is climbing at an estimated 25 to 30 percent annually. No taxi ride in Cairo is complete without a round of commiseration among passengers about rising prices. Housewives come back from the market mumbling about new increases.

"There are problems here that in many othere countries would lead to political instability," said a U.S. diplomat.

Aside from counting on the Egyptian people's tradition of long-suffering, the United States has since 1975 allocated about $4.5 billion to aid the economy, an amount that, per capita, is more than the Marshall Plan to rebuild Europe after World War II.

As a result, U.S. projects stretch across the country. American money and expertise are behind garbage trucks and water faucets in Cairo slums, unloading cranes in Alexandria harbor, concrete-lined irrigation canals in farmland near El Mansura and electric power equipment for Beni Suef homes.

Donald Brown, who heads the vast program, emphasizes that much of what is being done cannot be seen readily because it is part of a commitment to reorganize the whole cumbersome economy. This is seen as part of Sadat's desire to reorient Egypt from the socialism instituted under Nasser and abandoned in principle by Sadat's "open-door" policies.

But in addition, the aid is regarded as practical demonstration of what happens to U.S. friends and an attempt to support Sadat by administering to his economic Achilles heel.

"I think one needs to be very honest, because the level of this program is a politically determined level," Brown told a group of Egyptians recently. i

"We don't have the largest aid program in the world in Egypt just because we approve of Egyptian economic policies. We have it because we think that there are actions being taken by the Egyptian government in the political sphere in respect to broad Middle East issues with which we are in agreement."