Immediate inference is concerned with arguments made of a single categorical statement as a premise, and a single categorical statement using the same terms as a conclusion.

Conversion
Conversion deals with the exchange of subject and predicate. There are valid forms for E and O. (see general categorical statements for whate A,E,I, and O mean):
No S are P (E) therefore No P are S (E)
Some S are P (I) therefore Some P are S (I)

Contraposition
Contraposition means swapping the subject and the predicate, and complementing both. Contraposition is valid for A and O:
All S are P (A) therefore All non-P are non-S (A)
Some S are not P (O), therefore Some non-P are not non-S (0)

Obversion
Obverting a statement entails changing it from affirmative to negative or vice versa, and then replace the predicate with its complement. Obverting any statement is valid:
All S are P (A) therefore No S are non-P (E)
No S are P (E) therefore All S are non-P (A)
Some S are P (I) therefore Some S are not non-P (O)
Some S are not P (O) therefore Some non-S are not non-P (E)

Reference:
Harlan Miller, Philo 5505 course notes (at virginia tech)