Abarim Publications' online Biblical Hebrew Dictionary
The verb ירד (yarad) is a very common verb expressing a downward motion: to decline, descend or to go down towards some lower location or to sink into something, etcetera. This root seems to be related to the verb רדה (rada), meaning to rule or have dominion.
It's used in all expected ways, from a going down a mountain (Exodus 19:14) to the falling of hail (Exodus 9:18). But it also means a going away from a place of prominence, such as a palace (2 Samuel 11:9). Since one "goes up" to Jerusalem, going away from Jerusalem is described as a "going down" (Judges 1:9).
One who is humbled is made to "go down" (Isaiah 47:1). One who is defeated "goes down" (Hosea 7:12). Being sad is being down (Isaiah 15:3 — an expression surviving in English). And at death one descends into Sheol (Numbers 16:30).
The many references to the Lord 'coming down' (Genesis 11:5, Exodus 19:18) should not be confused with a physical descending from some high heaven in space, but rather his inclining toward an issue, or simply a natural consequence of some man-made situation (like the 'one descending from Jacob' mentioned in Numbers 24:19).
This verb's sole extant derivative, the masculine noun מורד (morad), means descent (Joshua 10:11) or hanging work (1 Kings 7:29).