It depends on what you are designing. If you're designing a small game for yourself, planning ahead isn't very important. If you're designing programming for the navigation module of a Martian probe that's going to be out of contact with Earth as it goes behind Mars, then it's kind of important that you get everything exactly right, and that you ask, for example, whether your function output should be in metric units or not.
No. And the reason is because whether something is important or not is an opinion — not a fact.
You can ask, "Is it true that my teacher thinks it is very important?" and then the answer is probably yes.
Keep in mind that your question doesn't say you are designing something for a client. You may be designing something for yourself.
Teachers and textbooks ask remarkably poorly-worded questions. Please point this out to your teacher.
Only newbies try to start writing code before they fully understand the requirements.
A better solution – 'Design Thinking'. It tries to make sure developers understand the problem, not the requirements.