It's not possible to check every possible condition in your software application. Testing techniques help you select a few test cases with the maximum possibility of finding a defect.
Boundary Value Analysis (BVA): As the name suggests it's the technique that defines the testing of boundaries for specified range of values.
Equivalence Partition (EP) :This technique partitions the range into equal parts/groups that tend to have same behavior.
State Transition Technique: This method is used when software behavior changes from one state to another following particular action.
Error Guessing Technique: This is guessing/anticipating the error that may arise while testing.This is not a formal method and takes advantages of a tester's experience with the application