With the help of Polish Communists, the Soviet authorities quickly managed to crush all the overt opposition. Combatants who were members of the AK ("Home Army") and WiN ("Freedom and Independence") independent undergound resistance organisations were killed, deported to Russia, or sent to prisons or labour camps; their leaders were imprisoned in Moscow and tried in a showcase trial. A condition was formally set by the Western Allies that democratic elections be held, but members and associates of Mikołajczyk's PSL independent peasants' party were arrested, intimidated, and murdered. The results of a referendum of 30 June 1946 and parliamentary elections in January 1947 were rigged. Mikołajczyk, Deputy Prime Minister of the Interim Government, fled the country. Thereupon Poland would be ruled by the PPR ("Polish Workers' Party," which in 1948 forcibly absorbed a pre-war socialist party and changed its name to PZPR, the "Polish United Workers' Party" ). Between 1948 and 1956, the Stalinist era, Poland was under the absolute rule of the PZPR Communist Party assisted by the secret police and "Soviet advisers". Repressive measures were directed not just against political opponents, but the general public. Former AK combatants and Catholic priests filled the prisons. Cardinal Wyszyński, Primate of Poland, was interned in 1953. Iinconvenient PZPR members (like Party Secretary Władysław Gomułka) were imprisoned too. Poland was a satellite of the USSR. There was next to no private business, and economic specialists were all Communists; there was an attempt to to collectivise the agriculture; and enforced industrialisation caused significant drop of the standard living and severe discontent.
Read about Michael Dobbs' novel "Churchill's Triumph" where you can find info on Yalta and the situation in Poland in 1945