The net ionic equation focuses only on the ions in solution that react with each other. The other ions that don't participate in the reaction are called spectator ions and are not included in the equation. First let's separate each reactant into its constituent ions since that is how they will exist in aqueous solution.
2H+ + (SO4)2- + Ca2+ + 2OH- --> ?
The 2 protons will combine with the 2 hydroxide anions to form 2 molecules of water. The calcium and sulfate ions will remain in solution since CaSO4 (calcium sulfate) is a compound that dissolves in water.
2H+ + (SO4)2- + Ca2+ + 2OH- --> 2H2O + Ca2+ + (SO4)2-
The calcium and sulfate ions are spectator ions since they remain unchanged and are eliminated from the net ionic equation. The net ionic equation is:
2H+ + 2OH- --> 2H2O
H2SO4 + Ca(OH)2(aq) --> 2H2O + CaSO4
full ionic equation:
2H+ + (SO4)2- + Ca2+ + 2(OH)- --> 2H2O + Ca2+ + (SO4)2-
net ionic equation:
2H+ + 2(OH)- --> 2H2O