a small focusing particle that singles out its clause, either to insist on it (surely, indeed) or to fence it off from what came before (only, but, however)
This is one of Biblical Hebrew's short pointing words. It sits at the front of a clause and puts a spotlight on what follows, but the spotlight does two opposite-looking jobs depending on the context, and a translator has to feel which one is meant.
In one use it pushes forward, underlining the statement as certainly true: surely, truly, indeed. A speaker who is sure of something, or who has just realized it, leads with this word, as when Laban tells Jacob, surely you are my own flesh and blood, or the psalmist declares, surely God is good to Israel.
In the other use it pulls back, carving out an exception or a limit against the surrounding flow: only, but, however, except. The flood wiped out everything, only Noah was left. You may eat the animals, only these few you must not eat. The same little word that can ring with certainty can also quietly draw a boundary.
The two readings are not separate words but two pressures inside one particle: it always isolates and intensifies its clause, and whether that comes across as emphatic affirmation or as a narrowing exception is decided by the sentence around it. A handful of verses, especially in poetry, can be heard either way, and translations openly disagree about them.