Adverbs such as "usually" or "initially".

§1. Inform uses the following scale to measure how certain it is that something is true: 

define IMPOSSIBLE_CE -2
define UNLIKELY_CE -1
define UNKNOWN_CE 0
define LIKELY_CE 1
define CERTAIN_CE 2

§2. A special certainty level is used for a temporal sense of certainty: 

define INITIALLY_CE 3

§3. 

void Certainty::write(OUTPUT_STREAM, int level) {
    switch (level) {
        case IMPOSSIBLE_CE: WRITE("impossible"); break;
        case UNLIKELY_CE: WRITE("unlikely"); break;
        case UNKNOWN_CE: WRITE("(no certainty level)"); break;
        case LIKELY_CE: WRITE("likely"); break;
        case CERTAIN_CE: WRITE("certain"); break;
        case INITIALLY_CE: WRITE("initial"); break;
    }
}

§4. Certainty adverbs are found mainly in regular sentences:

A door is usually open.

They are syntactically legal in existential sentences too, though in English this usually expresses emphasis rather than a measure of probability: consider "there certainly are men in the room". Inform allows this, in any case. In conditions, Inform is more picky. For example, in assertions one can write

A box is usually closed. (1)

but in conditions one can't write

if a box is usually closed, ... (2)

This is because (1) is essentially a statement about the future, not the present or the past, whereas conditions like (2) must always be determinable at once: run-time code cannot know what will generally happen, only what is now the case and what has been the case in the past. 

<certainty> ::=
    always/certainly |  ==> { CERTAIN_CE, - }
    usually/normally |  ==> { LIKELY_CE, - }
    rarely/seldom |     ==> { UNLIKELY_CE, - }
    never |             ==> { IMPOSSIBLE_CE, - }
    initially           ==> { INITIALLY_CE, - }