Mood Adjuncts [...] relate specifically to the meaning of the finite verbal operators, expressing probability, usuality, obligation, inclination or time. They therefore typically occur next to the Finite, either before or after it; for example 'she probably hasn’t arrived', 'he doesn’t always hear', 'you certainly must go', 'I’d gladly help', 'she’s already arrived'. There is also another set expressing intensity, e.g. 'just', 'simply' as in 'you simply can’t tell'.
- Halliday (1985), a pag.82