This can vary depending on the style guide and the country, but generally in the US, it's acceptable to put the period inside the quotations. (British English often places it outside.)
Exceptions:
If you're including a citation, though, the period will come after the parenthetical citation and therefore outside the quote marks.
If you're creating dialogue with speaker tags, the period will likely need to be a comma.
Other punctuation, like question or exclamation marks, are pickier: they go inside the quotes if the original quote had them, but outside if it's your own question or exclamation.
Examples:
- "I don't know what to tell you."
- "I don't know what to tell you," he said finally.
- Climate change is "not the end of the planet, but the end of world" (Green 87).
- Author Jaime Green has written several articles about space and alien life, including one for The New York Times Magazine called "Befouling the Final Frontier."
- What can we possibly do if this is "the end of the world"?
- To determine the origin of life, we must first define life, asking questions like, "Is life self-replicating information?" (Green 63)
Quotes pulled from:
Green, Jaime. The Possibility of Life: Science, Imagination, and Our Quest for Kinship in the Cosmos. Hanover Square Press, 2023.
