Which sentence is correct ? 1) When there are no clouds, then there is no possibility to rain. 2) If there are no clouds, then there is no possibility to rain. Do these sentences have a difference in their meaning at all ? thanks in advandace
These two have similar meanings, but there are subtle differences.
When there are no clouds...
implies that there will be (or have been) occasions when there are no clouds, at some time or other.
If there are no clouds...
does not imply that, and means only that there is the possibility of no clouds.
This is illustrated by the conversation cliche:
Alice: If I graduate from school....
Bob: You mean when you graduate from school...
The first implies an uncertainty as to whether the graduation will ever happen. The second implies confidence that it will happen sometime.
'When' also implies simultaneity, and would not be used when cause and effect are distant in time. You might say "If I get on the plane I will arrive in Europe" but not "When I get on the plane I will arrive in Europe."
Fundamentally 'when' is about time and 'if' is about condition. For practical purposes those are often the same, because the 'condition' is true at specific 'times'. If that is not the case then you need to distinguish between them.
In response to a comment as to whether 'if' or 'when' implies habit, neither does. However the use of the present continuous tense, e.g. "If I watch TV I eat popcorn" implies repeated occurrence. Substituting 'when' for 'if' does not change the meaning. If you changed the tense e.g. "If I watch TV I will eat popcorn", that would no longer imply repeated occurrence, but would talk only about an occasion in the future.