grammaticalise

verb3

1. •
   To make grammatical.

   • Enhanced output arises when learners grammaticalize their output either through the use of more advanced interlanguage forms or of target language forms. — 1999
2. •
   To integrate into a system of grammar; to make (something such as a constraint) an element or rule of grammar, to cause (something) to be required by grammar.

   • ... Udmurt, Turkish, and Yucatec Mayan to test (and critique) the hypothesis that possessives grammaticalize into definite articles. — 2003
3. •
   To cause (a word, a suffix, etc) to undergo grammaticalization.