There is a clear-cut difference in the usage of  IN CASE and IF. In the case of ‘In case”, something is being done in anticipation of something else. Similarly we did ( or not) something  anticipating something else.  But in the case of ‘IF’ , nothing is being done anticipating something else.  We do something only if a need arises.  When ‘In case ‘ is used in the past , it explains why we did something in the past.

IN CASE and IF compared.

• You may keep the file with you in case the Director asks for more details.( here  one is being advised to keep the file with them which may or may not become necessary).
• If the Director asks for more details, you may come and collect the file from me. ( in this case, one is not taking the file with them in advance).
• I kept the file with me in case the Director asked for more details.(we did something in anticipation of something else and whether it happened or not is not explained here )
• We will book one more room in the hotel in case the Director wants to stay.( we are not sure whether the Director will stay or not, but we have decided to book a room for him)
• We will book one more room if the Director wants to stay.( here the booking of room will be done only if  the Director wants to stay.)
• I will give you my phone number in case you need to contact me.( here the phone number is being given in advance to be used in case of a necessity)
• I gave him my phone number in case he wanted to contact me( past situation ).
• I will send you a helper if you need his help. (i.e you can ask for a helper only if such a need arises.)

Note :  However, in certain situations ‘ IN CASE’ is seen used in place of ‘IF’.

• In case I forget please remind me of the gift to be purchased .( here  in case= if)
• You can phone me in case you have any problem.( here  in case= if)