My uncle is going to sponsor me for my trip and he is paying for all my accommodation. The question asks that how much I would be spending on my UK trip. Should I put the complete amount (about 1500 pounds), which includes the money my sponsor is also paying, or only include how much I would spend from my own money as an answer to this question. Moreover, there is also a separate question asking me if someone else is paying for your trip.. Another questions asks 'how much money do you spend each month'. I am a student and entirely dependent on my parents. Hence, should I write an estimated amount that my parents pay for my education, food and other things per month?
Might be worth reading the answer to a similar question asked by myself a few months ago, as well as my comment/response to the answer in which I detailed what I had entered for those questions (my visa was successful). There should be separate questions asking how much money you have to spend and how much of that money is contributed by yourself.
This should answer your question itself, but if I may also direct you to this answer, you might find helpful info there too. Remember, you must show your uncles financials, including bank statements, proof of earnings and a letter from him expressly stating that he will be sponsoring you, and why he is sponsoring you.
Note: According to your question, your uncle is only sponsoring your accommodation, but at the same time, you're an unemployed student. Naturally concerns will arise as to how you'll be paying for the rest of the trip. Surely, there has to be another sponsor? Your parents? If they are sponsoring you, you'll have to list them as financial sponsors as well, with the same details as in the paragraph above. If they aren't then you're going to have a hard time proving that you're able to afford paying the non-accommodation costs yourself. I wouldn't recommend that.
You may have to decide this for yourself but in doing so it might help to consider why such questions are being asked - despite the fact that for government forms without many guidance notes as to what is required this is a dangerous thing!
should I write an estimated amount that my parents pay for my education, food and other things per month?
No. The question is how much money do you spend each month, not how much are you costing someone else each month. At an extreme, should someone give you an exceedingly expensive present it may have cost them a lot but that is nothing that you have spent and makes no difference to your financial standing in UK and ability to subsist without resort to state funds. (Assuming you don't bring the present with you, pay duty and tax as appropriate and then sell it for a good price.)
Should I put the complete amount (about 1500 pounds), which includes the money my sponsor is also paying, or only include how much I would spend from my own money as an answer to this question.
Yes, put the complete amount. Here I think the opposite applies. Should your sponsor fail to pay for any reason these are costs that you would or might incur, hence reduce your financial ability to cope with other eventualities. So show your estimate of the costs for your trip, regardless of who ends up footing the bill.
there is also a separate question asking me if someone else is paying for your trip
I think this effectively 'balances out' your concerns about the £1,500. If your sponsor is prepared to contribute say £1,000 towards the total cost then, taking the answers together and assuming you make clear what you are doing, it is evident that the decision regarding finances is "Can visitor afford £500 on their own AND can sponsor afford a contribution of £1,000, OR can visitor afford the entire £1,500 if there is doubt about the sponsor's financial standing".
Both of you should provide suitable evidence (eg recent, original bank statements) as necessary to establish the same thing, but to a different extent each. Both cases should preferably show liquid assets of substantially more than just the expected expenditure. In addition, you should provide a very good reason for your sponsor being prepared to contribute anything at all (such as being a close relative).