we use 사용하다 with some products or machines, some things that are exist in reality. 쓰다 has same meaning with 사용하다 and 쓰다 is more commonly used. for example can l use this laptop? 이 랩탑 써도될까요? 이 랩탑 사용해도 될까요? 이용하다 is used with service , place , some thing that you can't see such as power , technology or pure heart or kindness etc. for example I use music to communicate with them 나는 사람들과 교감하기 위해 음악을 이용한다. Bus use is increasing among the staff. 직원들 사이에서 버스 이용이 잦아졌다. we use 이용하다 with transportation since bus or train can be also a service.

Hi Birthe. This is very tricky questions. And sometimes you can use both. Technically, 이용 means you utilize something for your convenience and use it for something good. And 사용 means you utilize something with the purpose based on its function. You can use 쓰다 in many cases. 쓰다 here is not writing but utilizing. So if you use it for something good, then 이용. Or if it's more of usage or utilization, then 사용. Honestly, it's hard to explain it and not sure my explanation is right haha. But you can give me some examples, I'll check them.

이용하다 : 대상을 필요에 따라 이롭게 쓰다.(utilize, take advantage of, use beneficially) 사용하다 : 일정한 목적이나 기능에 맞게 쓰다.(use) 쓰다 has same meaning as 사용하다. 쓰다 is mostly used colloquially. 지하철을 이용한다(O) 지하철을 사용한다(?) 이용한 물건은 제자리에 꽂아 주세요.(O) 사용한 물건은 제자리에 꽂아 주세요.(O) 어른에게 존댓말을 사용한다(O) 어른에게 존댓말을 이용한다(?)

If you use 사용 where 이용 was supposed to be used, and yet it would be acceptable even though it would get an awkward moment. But because 이용 has an image of taking advantage of, if you misuse this it would be inadequate in many ways. Still most of the natives would understand what you have to say.