I went to the UK to study after 12th? But I left College and I came back to India before my visa expired. I continued my study in India and started working. I want to apply for Canada now but I don't want to mention about the UK Visit as it will affect my application. What can happen? My passport is clean as I lost the last one which had my UK visa in.
You must answer all questions truthfully. Australia, Canada, New Zealand, UK and USA share immigration data https://en.wikipedia.org/wiki/Five_Eyes; while the extent to which they do this is unknown, you should assume that Canada will know, or can easily find out, about your previous UK visa. Having a ‘clean’ passport makes no difference - your visa history can be traced via biometrics, for example.
You don’t say why you don’t want to disclose your UK visa, from which it seems reasonable to assume you have something to hide. If you tell the truth, there’s a chance it may not affect your application. If you lie, there’s a good chance it’ll be discovered, and your application will be refused. You’ll then have a black mark on your immigration history that will be difficult to overcome, plus a possible ban for deception. If you think whatever it is you’re trying to hide is sufficiently serious to warrant lying in a visa application - don’t apply.
I want to apply for Canada now but I don't want to mention about the UK Visit as it will affect my application.
Does the visa application give you a "tell the truth" option and a "tell a lie" option?
If there is no such option given, the immigration officers expect that you are telling the truth. If they catch you lying you will likely never be able to travel to that country in your entire life.