Speak
Like "say," the word "speak" refers to thoughts and feelings moving from our more internal spiritual levels to our more external ones -- and ultimately from the Lord, who is in a sense the most internal spiritual level of all. This is generally called "influx" and "perception" in the Writings, meaning they are thoughts and feelings that flow in in a complete way from the Lord, rather than being things we have to think about and figure out. On a number of occasions "speak" and "say" are used together; in these cases "speak" refers more to intellectual instruction in matters of thought and "say" refers more to feelings and affections that flow in directly.
Jeremiah 21:7-8
7
Afterward, says Yahweh, I will deliver Zedekiah king of Judah, and his servants, and the people, even such as are left in this city from the pestilence, from the sword, and from the famine, into the hand of Nebuchadnezzar king of Babylon, and into the hand of their enemies, and into the hand of those who seek their life: and he shall strike them with the edge of the sword; he shall not spare them, neither have pity, nor have mercy.
8
To this people you shall say, Thus says Yahweh: Behold, I set before you the way of life and the way of death.
