어서

so, because; and then

Joins two clauses where the first leads into the second — either as the reason ("it was so hot, so climbing was hard", "it isn't ripe at all, so it's like a rock") or as the step that comes first ("study it and then tell you", "thank you for watching"). It fuses onto the verb stem, so the joint is often invisible in the middle of a word. Compare 고, which lists two things with no link implied, and 니까, which also gives a reason but is the one used before a command or a suggestion.