For instance, it's technically incorrect to start a sentence with the word "because," even though any clause that you could legitimately begin with the word "because" is grammatically and syntactically acceptable at the beginning of a sentence.
Because language is complex, teachers often simplify with easy to remember 'rules of thumb' which are technically not true, but are 'good enough'.
It is perfectly acceptable to start a sentence with 'Because', as in the above sentence. The rule is formulated to help avoid a certain trap. Because it's an easy trap to fall into. It is much easier to say 'avoid starting sentences with 'Because'' with a view to the student eventually learning why this is the case and learning when one can correctly start a sentence this way.
It is tricky to explain independent clauses and dependent clauses to someone and that if you use a dependent clause in a sentence, there needs to be an independent clause for it to make sense.
"Because I don't like the rule."
"Although the rule is arbitrary."
The sentences start with dependent clauses and they need something else to make sense.
"Because I don't like the rule, I often break it."
"Although the rule is arbitrary, it has its uses."