Advice on how to ask good questions during talks
1) How many talks should I attend and why?
Being broadly educated in sciences and within your own relatively large field (e.g., astrophysics) remains an important goal of graduate training. So one reason to attend talks is to develop and maintain a broad knowledge of the field.
Another reason is to improve your own speaking skills. You will be asked to present your research at your GBO, at your thesis defense, at job talks and at innumerable conferences. By attending talks by others, you will develop a feeling for what is and isn't an appropriate level of presentation in different settings and learn to design your presentation to target a particular audience. A 15-min presentation at a topical conference is quite different in spirit from a colloquium at a joint physics and astronomy department.
During the academic year, averaging one-two full-length talks per week in your broad field (e.g., astrophysics) is a good target goal during graduate school. For example, regularly attending the CAS seminar and the joint JHU/STScI astrophysics colloquia is a reasonable expectation. Attending additional occasional talks in other areas of science is also beneficial, but may be prohibitively difficult with many competing demands on your time.
2) Why do you insist that we should practice asking questions at the talks?
If you plan to ask a question, you are likely to pay much more attention to the talk and to remain 100% focused on the presentation. Developing skills for asking questions is directly relevant for developing better public speaking skills and for being able to "think on your feet". Finally, designing your questions for the speaker will enable you to understand what kinds of questions you might be asked when it's your turn to present.
3) What if I can't come up with a question?
If you've sat there and paid attention for 45 minutes, you can find a question to ask. If the speaker lost you on slide 2, ask a question about that slide. If the speaker demonstrated that 2+2=4, then ask them about future work directions. Think deeply, formulate your question in your mind and raise your hand.
3) Good practices for asking questions:
-- Never interrupt the speaker in the middle of a sentence. If you are asking a question during the talk, raise your hand and wait to be called on by the speaker, or wait for a natural pause. If you are asking a question at the end, raise your hand and wait to be called on by the moderator.
-- Be respectful of yourself. Do not apologize for asking the question. Do not preface your question by saying "this is a stupid question, but". Just raise your hand, smile and ask your question clearly and concisely.
-- Be respectful of the speaker and do not undermine their presentation. For example, "Have you read the paper by Smith et al. 2020?" or "You might not know Smith et al. 2020 paper but..." are both rather disrespectful and hostile ways of asking a benign question. A much better way of asking this is to say "Smith et al. demonstrated that A is proportional to B based on X technique, but you are showing that A is B^2, can you comment on what might be different?" With this approach not only are you not putting the speaker on the spot about their familiarity with Smith et al., but you are also respectful of the audience by explaining Smith's technique.
-- Do not lecture the speaker or use the question as an opportunity to present your own work. The time and place to do this is at your own presentation, not somebody else's. If you really want to draw parallels or contrasts with your own approaches, use the same approach as in the paragraph above.
-- If you are not getting the answer, or if the speaker didn't quite understand your question and answered a different question, do not press. Leave longer discussions for later. "I'd be eager to talk about this later" is an excellent way to wrap up a question that's getting out of control.
4) Good and best questions
-- Clarification questions (what is on the axes? what's the difference between the solid and the dashed line?) are fine.
-- Asking to contrast and compare different methods and different results can be quite illuminating.
-- Generating an interesting new idea for some calculation or test and asking the speaker if it's worthwhile could be the best, though see above on not lecturing the speaker (in 95% of cases they have already thought about it, and in the 5% of the remaining cases you want to sound like a prospective collaborator, not hostile audience!).
5) When you are asked a question during your talk
The first and most important rule is: listen carefully until the end of the question. Everything else is really a function of how well you know the landscape of your presentation and how much experience you have. But even speakers with lots of experience often make the mistake or failing to listen until the end of the question and jump to a (frequently incorrect) conclusion that a different question is being asked. So again: listen patiently until the end of the question.
Having heard the question, take a deep breath and think about the most concise answer. If the question is easy, answer simply and to the point. If the question is difficult, there is no point in wasting everybody's time talking about things you know little about. Say something relevant and brief and ask for an in-depth discussion with the asker for later.
Only go for a long answer if you already have well-organized, interesting thoughts about that exact topic.