If A then B means A is contained in B.
A, therefore, acts as a sufficient condition, while B acts as required condition.
Various sentences that can be interpreted as If A then B are
1) A is also B
2) A only if B
The contrapositive of the above is
If
not B, then
not A.
Another statement that means slightly different is
A unless B.
This means: If
not B, then A.
Venn Diagram: Use of circles for event A and event B can easily simplify the statements of the kind of If then'.'