I found that sometimes things just need to be explained in a different way so here is my shot at it:
Whenever you have a Many-to-Many (M:M) relationship between two entity types, that relationship table should have a foreign key that refers back to both entity types. For example, in ER Exercise #1, the "Wrote" relationship table will have a Foreign key that will refer back to both Paper and Author.
Whenever you have a One-to-Many (1:M) relationship, the table for the "Many" entity type will have a foreign key that refers back to the "One" entity type. For example, since one journal can have many issues, the Issue table should have a foreign key that refers back to the Journal entity type.
Another way to remember this is to ask, "Which way would not require adding extra records?" To answer this, picture each table without the foreign key. When you try to add a foreign key field(s) to the Journal table (the "One" entity type), you would also have to add additional records (rows) since one journal can have many issues. This causes repetition, which is a violation of 1st Normal Form. However, if you add a foreign key field(s) to Issues, all you add is that field(s). The number of records remain the same.
I hope this helps.
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