The biological sequences that occur in DNA, RNA and proteins can be considered as strings formed over the well defined chemical alphabets. Such gene sequences form structure based on the complementary pair and the structures can be interpreted as languages. Matrix insertion-deletion system has been introduced a few years back that modelled several bio-molecular structures occur at intramolecular and intermolecular level. In this paper, we identify some structures that are frequently noticed during RNA folding process such as double bulge loop, extended internal loop, triple stem and loop and we give the corresponding formal language representation to such structures. Further, we model the structures using Matrix insertion-deletion systems. This work is pioneering to give the language representation and modelling the structures of RNA folding process using formal grammar. © 2017, Springer International Publishing AG.