Questions & Answers about Мой друг часто присылает мне сообщения вечером.
Мой друг is in the nominative case, because it is the subject of the sentence – the one performing the action (sending).
- Кто? Что? (Who? What?) → мой друг (my friend) → nominative.
- Присылает кого? что? (sends whom? what?) → сообщения (messages) → direct object (accusative).
Моего друга would be in the genitive (or accusative masculine animate), which you’d use, for example, after certain prepositions or verbs, or as an object, not as the subject:
- Я жду моего друга. – I am waiting for my friend. (Here, моего друга is the object.)
In this sentence, мой друг is clearly the subject, so nominative is required.