Depending on their domain of operation, FG [Functional Grammar] recognizes the following types of operators: (2) a. Ω : term operators b. π1 : predicate operators c. π2 : predication operators d. π3 : proposition operators e. π4 : Illocutionary operators Some properties which distinguish operators from predicates are the following: • operators are grammatically rather than lexically expressed. By grammatical expression we mean expression through items belonging to closed paradigms, manifesting themselves in the inflectional modifications of lexical predicates, and/or in “form words” such as articles, quantifiers, auxiliary verbs, etc. • operators typically capture a limited number of crucial distinctions in some semantic domain, such as: (3) ‘DOMAIN’ ‘DISTINCTIONS’ NUMBER singular-dual-plural TENSE past-present-future POLARITY positive-negative ASPECT-1 imperfective-perfective ASPECT-2 prospective-progressive- perfect MOOD possible-necessary ILLOCUTION declarative-interrogative- imperative […]. • operators have “scope”, i.e. they extend their influence over a certain section of the underlying clause structure, depending on the level of structure where they operate.
- Dik (1989), Pag. 137-138

Operators are used to capture those modifications and modulations which can be brought about at the relevant level by grammatical means […].
- Dik (1989), Pag. 50