Mixed conditionals explained.

Mixed conditionals are called 'mixed' because they use two different conditionals.

Compare these sentences from SB p.68, 5c

1a If the kidnapper hadn’t licked that envelope, (Past = 3rd conditional) he wouldn’t be in prison now. (Present = 2nd conditional)

1b If the kidnapper hadn’t licked that envelope, (Past = 3rd conditional) he wouldn’t have gone to prison. (Past = 3rd conditional)

2a If they weren’t such good actors, (Present = 2nd conditional) most of them would probably have been found out much earlier. (Past = 3rd conditional)

2b If they hadn’t been such good actors, (Past = 3rd conditional) most of them would probably have been found out much earlier. (Past = 3rd conditional)

The mixed conditonals are 2nd + 3rd, of course.