When and why did poland change its government structure?

Currently the government of Poland is a democratic parliamentary republic. After the end of the Second World War, it was a socialist state. In the late 1980´s a movement of opposition to the government, headed by the trade union Solidarność and Lech Walesa, led to the establishment of the multiparty system and the return to capitalism. The modern Polish State arise and it was renowned: Republic of Poland.

whose power in the senate is equal to the power of the speaker of the house?