Halliday & Matthiessen (1999: 520-1):
The third type of expansion is one of enhancing the first process by another one setting up a specific semantic relationship, of which the principal ones are time, cause, condition, concession and means. Here again the grammar deploys a range of different conjunctions, which mark either the enhancing clause (when, because, by, though, if and so on) or the one that is being enhanced (eg then ‘at that time’, then ‘in that case’, so, thus, yet).