Is there a place where questions are grouped by subject?

Search is your best option.

You can click the topics (those buttons that say Fluther, questions, search, and topics at the bottom of your question, for example) to see more questions about that particular subject.

If you want to make sure you cover the bases, then use @Mariah & @Judi‘s methods. The topic buttons are great, but whether one gets assigned to a question or not is entirely up to the user asking the question, and what topics they think of and find are included in the standard selections.

On the right side of the Home page, you should see a list of the Popular topics immediately below the Community Feed.

If you click on one of the topic buttons, it’ll take you to a page showing all of the questions for that topic.

Topics are really the only grouping by subject. Another feature you might find useful is the “Related” column on the right hand side of every question page. Here you find a short list of a few similar questions. Although be warned, they are often very old so any posts you add to them may not be seen by anyone. It’s often better to re-ask a very old question.