Looks are deceiving here, as this APPEARS to be a very simple question. But in fact, there are many differences between these two words, both in meaning and usage, so I would suggest that you study the definitions and examples for both words in an online dictionary such as
www.longmanwebdict.com/ or
dictionary.cambridge.org (both give many detailed examples to help you understand the differences; pay attention especially to the context of the examples).
“In” is a preposition used with periods of time (e.g., a day, months, years, seasons); another common meaning is a container, place or area, or surrounded or enclosed by something.
'Inside' can be used as a noun, adverb, preposition or adjective. When it is used as a preposition, its meaning may be very similar to the positional/locational meaning of 'in' ('The employees are all in the office today.'), but 'inside' with a positional meaning usually has the added meaning of being in the inner or internal part or completely surrounded by the container or place ('They warned citizens to keep their clothes inside the building, as hanging clothes outside is forbidden.
Please note that 'in' (as a verb particle) can be followed by a gerund, whereas 'inside' cannot be followed by a gerund. Some other prepositions that are not followed by gerunds are: down, off, out, up, along, behind, inside, onto, under, opposite, and underneath.
I'd also suggest you do a collocation search on 'in' and then 'inside' at a site such as
www.edict.com.hk/concordance/WWWConcappE.htm to study the examples of how these two words are used in real writing.
