Before copying resources created by others, we must be certain of being legally authorised to do so.
Copyright laws depend on the country. We must be properly informed when we use foreign resources.
With Spanish resources, we should read the law Real Decreto Legislativo 1/1996, de 12 de abril, por el que se aprueba el texto refundido de la Ley de Propiedad Intelectual.
When no other license applies, we must consider every work published in the Internet as a work with Copyright.
Some authors publish their works with licenses that allow greater flexibility of use and reproduction. For example, by Creative Commons licenses or GNU. In such cases we must read the conditions of the license and be strict in their correct use.
Having a book with the text "photocopiable" does not give us any right to reproduce it in the Internet unless clearly specified. Teacher books given by the editors are usually aimed for their use with our students, not to be openly shared.
Public domain works can be used freely. It is easy to find them in the Internet. For example, we can download books that no longer have copyright from Project Gutenberg. However, we must accept the conditions in the license of use that this web gives to these works, because eventhough the content may be public domain, the edition of the texts is recent. Likewise, althoug a Baroque piece is public domain now, its performance by today's musicians may be subject to copyright.
In any case, be it to tribute their merit, to be a model for our students, or to prevent wrong attribution, it is always advisable to keep on quoting the original authors.