to add: to join more onto what is already there, and so to increase it or do it once again
At its root this word means to put more onto an existing amount. From that single idea two everyday uses branch out, and they look quite different in English even though Hebrew feels them as one. The first is plain addition: piling more onto a stack, increasing wealth, learning, or pain, tacking words onto a command, or paying back a debt with an extra portion.
The second use turns that same idea onto an action rather than a thing. When this word is paired with another verb, it means to do that action one more time, to keep doing it, or to do still more of it. Hebrew has no tidy word for our adverbs again, more, or further, so it reaches for this verb instead: it literally says a person added to speak, added to send, added to take a wife, and means spoke again, sent once more, married yet another.
A fixed oath grows out of that second use. Someone swears may God do so to me and add to it, calling down extra punishment on himself if he breaks his word. The verb here is not really about quantity at all; it is the engine of a stock curse that seals a promise.
Because one Hebrew word covers add, increase, do again, and the oath, an English Bible has to choose a different rendering almost every time, which can hide that a single idea stands behind them all.