Prime Minister Pierre Trudeau and premiers of Canada's 10 provinces ended a two-day conference here today unable to reach agreement on key constitutional questions.

Only modest agreement was reported on changes in the country's constitution. But Quebec Premier Rene Levesque, who advocates secession for his French-speaking province, left the conference saying the changes did not go far enough to meet Quebec demands for autonomy.

Quebec's intransigence on the question of constitutional change and the stubboorn defense of provincial control of natural resources by several western provinces could give Trudeau the issue he is seeking for the general elections he must call in the next 10 weeks.

Where agreements were reached, they tended to weaken the federal government at the expense of the provinces. This shift of power, which has been under way for about 15 years in Canada, is designed to accommodate regional and linguistic tensions which have become especially acute in the last five years.

Levesque, who has promised a referendum on what he calls sovereignty-association for Quebec within the next 18 months, tangled several times with Trudeau over the role of Quebec in the Canadian confederation.

In particular, Levesque opposed efforts to find a suitable amending formula for the constitution, an issue that has plagued federal provincial conferences in Canada for 51 years.

At present, most of Canada's constitution rests on an act of the British Parliament. Changing the document requires the unanimous agreement of the federal and provincial governments.

Trudeau has been trying since he became prime minister in 1968 to bring the constitution back to Canada with an acceptable amending formula. But Levesque repeated yesterday that Quebec would never consent to a formula until it gets all the powers it wants from the federal government.

Trudeau has been painting himself recently as the only political leader capable of holding Canada together against the strains of linguistic and regional discontent. He appeared generally conciliatory during the conference, preferring to allow the provincial premiers to attack Ottawa so as to keep open his option of running the next federal campaign on an anti-provincial platform.

The federal and provincial governments did reach some modest agreements. They agreed to give the provinces full control of family law, to allow provincial governments to license cable television operations, to create joint federal-provincial committees to oversee fisheries policy and to give the provinces a voice in appointments to Canada's supreme court.

But the conferees could not agree on reforming the Canadian Senate, now appointed entirely by the federal government. Nor could they agree with Ottawa's plan to entrench a charter of basic rights and freedoms in the constitution.