 This is called "decategorialization," or "morphological reduction".For example, the demonstrative 'that' as in "that book" came to be used as a relative clause marker, and lost the grammatical category of number ('that' singular vs. 'those' plural), as in "the book that I know" versus "the things that I know".Phonetic erosion (also called phonological attrition or phonological reduction), is another process that is often linked to grammaticalization.