مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is only a writer.
إِنَّمَا زَيْدٌ كَاتِبٌ
Zayd is indeed only a writer.
الْكِتَابَ قَرَأْتُ
The book—I read it.
The first uses negation and exception, and the second “innamā,” to express restriction. The third fronts the object, which may imply exclusivity in an appropriate context but can also simply topicalize the book. The distinction depends on the discourse setting.