You can make problems with multiple parts behave in different ways. You can ask Webwork to give or not give partial credit if some of the answers are correct, and to tell or not tell students which particular answers are correct.
For many problems, you may wish to give partial credit. For others, such as true/false & multiple-choice questions, you may wish to only give students credit if they get every part correct. The default setting is to give partial credit. To change this, you must install a different problem grader:
Insert the line
install_problem_grader(~~&std_problem_grader);
before the answer evaluation occurs, e.g. in the problem set-up area. If you do this, do consider having Webwork not telling students which particular answers are correct!
See http://webwork.maa.org/wiki/WeightedGrader for more advanced techniques, such as weighting some parts more heavily than others.
Independent of whether you give students partial credit on a problem, you may wish to tell them which parts are incorrect on some problems. For others, such as true/false & multiple-choice questions, you may wish to not tell them this until they have every part correct. The default setting is to not tell students which parts are incorrect. To change this, insert the line
$showPartialCorrectAnswers = 1;
before the answer evaluation occurs, e.g. in the problem set-up area. Setting this variable to 0 instead will keep students in the dark about which parts are correct, until they get all parts correct.