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The following passage uses the phrase "This requirement is because". Is that correct usage?
Modularization is a key requirement to manage the size and complexity of large ontologies that can be replaced by a smaller ontology. This requirement is because existing ontology languages such as OWL do not support partial reuse of ontology and ontologies are growing to cover more knowledge in a specific domain.
No. "Is because" is not correct. It is considered to be a mixed conjunction.
school teachers tell you not to use this.
it does not sound very good. It sounds a bit redundant or blunt.
in speech it is a common way to give a reason for something.
Dad, why is the sky blue? Well, son, _it's because of the absorption of the light spectrum by the atmosphere and the bla blah blah...
So when writing you want to avoid it altogether. But in colloquial speech it's fine.
It's perfectly correct, but almost never used. To speak it, you have to put hard emphasis on "is" and pause very slightly between "is" and "because" to make it clear that "is" means "exists". Otherwise, it seems like you left a word out.
Just like you can say "Law exists because humans created it", you can say, "Law is because humans created it". Similarly, you can either say "This requirement exists because ..." or "This requirement is because ...".
