IF, WHEN,UNLESS and IN CASE

Present Simple for the future:

Look at the following sentences-: 

•  You'll be tired tomorrow if you work so hard today.
•  Tell me when you reach home.
•   I won't do anything unless you agree.
•  Write the address down in case you forget it.  

We use the present simple for the future after linking words such as if, when, unless and in case.

IF or WHEN

IF

We use if for something that we think might happen.

• If you hear any news, can you inform me immediately?  ( you might hear the news)
• I will probably visit your place if everything goes well.  ( everything might go well)

 We use if ( not when) for something imaginary or impossible.

•  If I were you, I wouldn't have hurt her. 

 WHEN 

We use when for something that we know will happen. 

• When you hear some news, can you inform me immediately? ( you will hear some news.)
• I will prepare dinner when I reach home.( I will reach home) 

 We can use if or when in contexts where they mean 'every time'. 

• If you swim, you lose weight.
• When you swim, you lose weight.

 IF and UNLESS 

If not means same as unless.

Look at the following examples: 

• If I don't wear my spectacles, I'm unable to see.
• Unless I wear my spectacles, I'm unable to see.
•  The doctor will be here if she isn't called to an emergency.
•  The doctor will be here unless she is called for an emergency.

IN CASE

• We use In Case to talk about something that should be done to avoid problem at a later stage.
• Take a blanket with you in case you feel cold.
• I booked my reservation in case I don't get tickets later on.
• I'll bring in the washing inside in case it rains.