Ag₂S  +  Al  →  Al₂S₃  +  Ag

The Al is alone ("single"). It trades places with the Ag.

This is Single Replacement!

SO₂  +  O₂  →  SO₃

The SO₂ and the O₂ COMBINE to form just one product: SO₃

H₂CO₃ →  CO₂  + H₂O

On the left side (before), there is only ONE reactant.

On the right side (after), we can see that the reactant broke apart into multiple products.

This is Decomposition!

CO   +   O₂  →  CO₂

Cu  +  H₂SO₄  →  H₂ + CuSO₄