Incorrect: «جَاءَ الْمُسْلِمِينَ»; correct: جَاءَ الْمُسْلِمُونَ — the subject is nominative.
Incorrect: «رَأَيْتُ الْمُسْلِمُونَ»; correct: رَأَيْتُ الْمُسْلِمِينَ — the object is accusative.
Incorrect: «حَضَرَ مُسْلِمُونَ الْمَدِينَةِ»; correct: حَضَرَ مُسْلِمُو الْمَدِينَةِ — the nūn drops in iḍāfa.
The incorrect sentences are teaching counterexamples, not quotations from the source.