An imperative verb directs an addressee to do something. For example, قُلْ means “say,” and اتَّقِ means “be mindful.” The subject is often understood from the verb rather than written as a separate noun.
In بَلِّغُوا, the ending واو الجماعة indicates plural addressees. دَعْ lacks that ending and addresses one person. Both are commands, but the number of people addressed differs.
The whole sentence determines the duty: conveying knowledge does not mean circulating unverified claims, and interceding does not mean supporting an injustice.