Question

When showing a character's thoughts are quotation marks used? And if so, single or double? And in normal dialogue, which is preferred, single or double?

Answer 1

When you are writing your character's thoughts (which is called introspection) -you do not use quotation marks. In a regular dialogue, on the other hand -you do use quotation marks.
E.g. for introspection (with a tag):
Perhaps she will answer me someday, thought Mark.
E.g. for a regular dialogue:
"Did you have a nice trip?" -she asked.
"Yes! Thank you, I loved it!" -he answered enthusiastically.

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