Question

Ammeter connected in parallel to the circuit burns out why

Answer 1

Ammeter measures value of current flowing in circuit, so current should flow inside ammeter to give proper result. And it has very low resistance to ensure the correct measurement of current in the circuit.
If it is connected in parallel across any load then all current in circuit will choose lower resistive path (i.e ammeter) to cause it's circuit to be damaged. Hence it is used in series.