(i) Reaction of ether with HI give an alcohol and alkyl iodide $(R- I )$, in which $- I$ is bonded with more substituted part of ether in case of presence of $3^{\circ} C$ -atom in that part of ether.
Thus, the given reaction occurs as follows:
(ii) In case presence of $3^{\circ} C$ -atom, the atom $-I$ combines via $S _{ N }$ l mechanism, as $3^{\circ}$ carbocation (formed during the reaction) is highly stable (more than $1^{\circ}$ and $2^{\circ}$ carbocations) and do not follow $S _{ N } 2$ mechanism due to steric-hindrance.
