• Unless is used followed by an affirmative sentence to express ‘if …not’
• Even if emphasizes that something is happening, is going to happen or happened whatever the condition.
• Only if makes the condicion more restrictive.
• Suppose / supposing, what if are more frequently used in conversation and replace ‘if’.
• As long as can be used instead of ‘if’ to express a condition.
• Provided/providing that is the same as ‘as long as’ on a more formal way.
• In case gives a reason more than a condition. We can use should after ‘in case’

*All of them are used as alternatives to if, practically with the same meaning and used in all types of conditionals.

Examples

If it rains, I will take my umbrella. 

I won’t take my umbrella unless it rains. 

Even if it doesn’t rain, I will take my umbrella.

Even if it rains, I won’t take my umbrella.

Supossing it rains, I will take my umbrella.

As long as it rains, I will stay at home.

Provided that it rains, I will stay at home.

In case it rains,  I will take my umbrella.

In case it should rain, I will take my umbrella.