The so-called NICE properties of auxiliaries serve to distinguish them from main verbs. NICE is an acronym for:
Main verbs do not exhibit these properties. For instance, when we form a question using a main verb, we cannot invert:
[John sings] in the choir ~*[Sings John] in the choir?
Instead, we have to use the auxiliary verb do:
[John sings] in the choir ~[Does John sing] in the choir?
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Among the auxiliary verbs, we distinguish a large number of multi-word verbs, which are called SEMI-AUXILIARIES. These are two-or three-word combinations, and they include the following:
get to
happen to
have to
mean to
seem to
tend to
turn out to
used to
be about to
be going to
be likely to
be supposed to
Like other auxiliaries, the semi-auxiliaries occur before main verbs:
The film is about to start
I'm going to interview the Lord Mayor
I have to leave early today
You are supposed to sign both forms
I used to live in that house
Some of these combinations may, of course, occur in other contexts in which they are not semi-auxiliaries. For example:
I'm going to London
Here, the combination is not a semi-auxiliary, since it does not occur with a main verb. In this sentence, going is a main verb. Notice that it could be replaced by another main verb such as travel (I'm travelling to London). The word 'm is the contracted form of am, the progressive auxiliary, and to, as we'll see later, is a preposition.