The correct grammar can be written as such:
* He was careless; thus, he failed the test. * He failed the
test because he was careless. * Because he was careless, he failed
the test. * Since he was careless, he failed the test. * He failed
the test since he was careless. * He failed the test, for he was
careless. * He was careless. He failed the test. * He failed the
test. He was careless.