Wah How to use "In case".

We use in case to express that an action is connected with something that might happen.

• The football referee wears two watches in case one of them stops.                               ( There was a possibility of one of the watches stopping. )
• I will take some food and drinks in case we get lost in the forest.                                     ( I will take some food and drinks before I go to the forest because I want to make sure that I don't get hungry or thirsty when I get lost in the forest. )
• It's a bit cloudy. You should take an umbrella in case it rains.
• You had better take a cellphone in case you need to contact me.
• I think I will wear gloves in case I get cold finger.
• We should leave early in case the traffic is bad.

Note: After in case we never use will even though we talk about the future.
         You should take an umbrella in case it will rains. is incorrect.

The different between in case and if.
    I will take an umbrella in case it rains. (Whether or not it rains, I will take it anyway.)
    I will take an umbrella if it rains. (If it does not rain, you won't take it.)

Wah How to use present perfect tense.

1. We use the present perfect tense for actions that started in the past and continue in the present which have not finished yet. 
    I have lived in Bangalore since 2001. 
    (I started living Bangalore since 2001 and I am still here.)
    I have worked here for 10 years. (I started 10 years ago and I am still here.) 
    John is a musician. He has already made a lot of beautiful songs.
    I've already fixed it. Don't worry. 
    She has never fallen in love. 

2. We use the present perfect tense to talk about actions that just finished in recently. Usually use with just or recently.
    Japanese chefs have recently found a new way to cook wasabi. 
    My aunt has recently got back from Manila.
    I've just had lunch. I cannot eat anymore.
    She has just bought a new car. 

3. We use the present perfect tense to talk about our experiences that we have until the present without saying when these things happened.

    I have been married three times.

    They have never worked with me before. 
    We haven't met your wife (yet).
    Have you ever been to Malaysia (yet)?
    She is the most beautiful women I've ever met.