Consider the reactions
(i) ${\left({\mathrm{CH}}_3\right)}_2\mathrm{CH}-{\mathrm{CH}}_2\mathrm{Br}\underset{ }{\overset{{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}}{\to }}{\left({\mathrm{CH}}_3\right)}_3\mathrm{C}-{\mathrm{OC}}_2{\mathrm{H}}_5+\mathrm{HBr}$
(ii) ${\left(C{H}_3\right)}_{2}CH-C{H}_{2}Br\underset{ }{\overset{C_2{H}_5{O}^{-}}{\to }}{\left(C{H}_3\right)}_{2}CH-C{H}_{2}O{C}_2{H}_5+B{r}^{-}$
The mechanisms of reaction (i) and (ii) are respectively

A strong nucleophile favours the $S_{N}2$  reaction and a weak nucleophile favours the $S_{N}1$ reaction.   First $S_{N}1$  reaction because ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$ is used as solvent which is a weak nucleophile.

Second Reaction is $S_{N}2$ reaction because ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}^{-}$ is strong nucleophile