�In� and �inside� are words that are more or less synonymous as
prepositions, and can be used in most cases interchangeably. There is a
subtle difference, however, which explains why �inside� cannot replace
�in� in phrases like �in the thick of it�.
�Inside� is generally used to contrast with �outside�; therefore, it
carries with it the strong suggestion of being physically enclosed. For
example, to say �We were inside Russia� sounds wrong, doesn�t it? This
is because there is sense of being surrounded by walls, or tangible
boundaries on all sides, when one says one is �inside� something. It is
almost strictly locative, which in grammar/linguistics refers to words
that indicate location. For example, to say �The clothes are inside the
cupboard� sounds correct.
On the other hand, �in� is used more generally; it carries no such
suggestion. To go �in� can mean simply to participate in something (I�m
in the group), or to be at a place (I�m in Chennai); it does not conjure
images of enclosures or walls, or being physically surrounded.
