«فتختلف صور الكلام لاختلاف الأحوال»
Forms of speech vary because communicative situations vary.
A form of speech is more than a change in vocabulary. Sentence construction may highlight one aspect of meaning over another. Sometimes naming the agent is appropriate; sometimes leaving the agent unmentioned suits the setting. At times an unembellished report suffices, and at others emphasis is needed.
A rhetorical adjustment must not distort the intended meaning. It should communicate that meaning accurately and in a form suitable for the situation.