Question

1. why the terminal voltage across the load decreases with increases in load on the transformer?

Answer 1

If we find thevenin equivalent circuit of transformer, you will find a resistance in the series of voltage source.

This resistance is a combination of winding resistance and reflected reactance of the primary source .

As load increases, load current increases.

This will increase voltage drop across equivalent internal resistance of the transformer. Hence load or terminal voltage of transformer will decrease.

The above explanations provide a sufficient answer.