These two conjunctions have almost the same meaning and most often either can be used.
However, it's usually safer to use because. The reason for that is that a clause introduced by for ( which we will call "for-clause") has a more restricted use than a clause introduced by because:
1 A for-clause cannot come before the verb which it explains:
- Because he was late, he didn't eat breakfast, (for is not possible.)
2 It's not possible to use a for-clause to answer a question:
- a) What made him do that?
b) Well, he did that because he didn't have any other choice., (for not possible)
3 A for-clause cannot come after not, but or any other conjunction:
- She didn't eat dinner, not because she wasn't hungry but because she is on a diet, (for is not possible)
4 A for-clause cannot repeat what has been already stated. It always has to include some new piece of information:
- He behaved really bad. She was angry because he had behaved very bad, (for is not possible.)
But
- She was angry, for she didn't know how to make him behave himself. (Here for is correct; because is also possible.)
But why the restrictions? Well, the reason for them is that a for-clause does not tell us why a certain action was performed - but gives a piece of additional information which helps to explain it.
Let's have alook at some other examples of for-clauses:
- Her grades were bad, for she learned very little.
- The dog wagged its tail happily, for it loved its owner.
- When I saw the painting I knew I had to buy it. For I've always been a great lover of art.
It's good thing to know that in speech a short pause is made before a for-clause. In written English this place is usually marked by a comma, occasionally, as shown in the last example above, by a full stop.
Note that because could be used in the above sentences also, though for is better.