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You might find this useful: https://en.wikipedia.org/wiki/List_of_countries%27_copyright_lengths
Generally, about 50 years after an author has died the copyright to their work becomes part of the public domain, meaning that anyone can use it. This does differ between countries through, so the above Wikipedia article will help in that respect.
If you don't know who the author is, then it would be prudent to make reference to the fact that the lyric or words have been lifted from an existing work - otherwise there's the risk of being accused of plagiarism. Stating that the author is unknown is a good way to cover yourself in the event that the work isn't out of copyright. The important thing is to highlight that you're using something that someone else has written and that you're not taking credit for having written it yourself.
Hope that helps!
Hannah from the WEbook Team