The article proposes a cognitive revision of sentence invariants with respect to the explication of main parts. Confusing `composition' and `structure' and bringing a morphological nomenclature of word classes to nominate invariants is a methodological problem. Extrapolation of terms from level to level adds contradiction to the categories and is a problem in the learning and acquisition of the language and its grammar. A cognitively correct sentence on this basis is one-component, subject or predicate, subject or predicate, but not one-component, because they are parts of the structure, not of the composition. The predicate sentence is always personal, but when it is expressed by a verb with a form in the third person only, i.e. in principle, there is no first and second position of the paradigm of this morphological category, it is subjectless.