Ordinary noun: زَيْد
جَاءَ زَيْدٌ — رَأَيْتُ زَيْدًا — مَرَرْتُ بِزَيْدٍ.
Translation: Zayd came; I saw Zayd; I passed by Zayd.
One of the six nouns: أَخ
جَاءَ أَخُوكَ — رَأَيْتُ أَخَاكَ — مَرَرْتُ بِأَخِيكَ.
Translation: Your brother came; I saw your brother; I passed by your brother.
The ordinary noun زَيْد shows vowel case signs, while this special construct form of «أَخ» uses letters as signs.