Why does the water leak from tank into toilet bowl constantly?
Because the float valve is leaking. When the toilet cistern fills with water to the correct level a float puts pressure on a valve usually with a ruber washer of some sort. Either there is not enough pressure put on the washer by the float to stop th flow of water or the washer has become worn or perished. it is easily replaced and parts are usually available at your hardware. Even if you can get the replacement part units are available to replace the whole refiling control.