What can a student do if he has problems with homework problems that he is unable to solve?
In higher education, the role of educators is to help facilitate students' learning. One of the main mechanisms for that is to provide students with high-quality feedback on their assessments. Simply delivering lectures is not very efficient.  Testing students using multiple-choice questions through automatic tools is also not very efficient. Even giving students a grade without explanation how it was produced is not very efficient. 

If you are struggling with a task, you should approach your lecturer/professor, show them your best attempt, and ask for feedback, specifically, an explanation what is incorrect in your attempt and what you should work on to improve your understanding of the subject. And no, "your answer is wrong because computer says no, now go away and try again until it says yes" is not good enough. Politely ask for clarification of the relevant material, not mechanics of the automatic assessment.