Kopernio - Installation guide see: https://kopernio.com/
Similar to the Library Access web browser extension. Kopernio searches through legal channels to see if a pdf is available online.
Unpaywall - installation guide: http://unpaywall.org/
Unpaywall refers you to the legal, author published and authorized version of the article.
These are publications published via the green route, Open Access (behind this link you will find more information about publishing Open Access, (in dutch) ).
Open Access Button - installation guide: https://openaccessbutton.org/
When you submit a scientific article, the Open Access button searches the internet for available PDFs. If it is not found, the Open Access button starts a request to the author.
More and more authors are available online. In addition, many authors can be accessed via platforms such as Researchgate, Academia.edu and Mendeley.
On Twitter you can use the hashtag #icanhazpdf in combination with the link of the article you can't access. Maybe someone will be able to send you the article: https://twitter.com/search?q=%23icanhazpdf.
At many university libraries you can request a day pass that allows you to consult and download online resources. Please bring your proof of identity with you. This is possible at the Radboud University in Nijmegen (university library), for example.
If you have not been able to find the full text despite all the tips and tools, please consult the information specialist of your course. He or she will be happy to help you.