"물건 and 사물" - 물건: an INDIVIDUAL physical object; an item, product, device, etc. - 사물: physical objects taken COLLECTIVELY (used mostly in writing, lectures, etc). So they are rarely interchangeable. 물건 is very widely used in daily life, while 사물 is almost never heard in conversation.

Yes. both are interchangeable to use in the specific situation. * 물건 is the same as 'thing(s)' and 'matter', both of which have physical forms. If you want to express 'matter' such as 'what's the matter?', use '일', '문제(problem)' * 사물 has the meaning of '물건', but sounds more formal. Yes, that's not used in colloquially, but in the field of law and philosophy. Unfortunately, there's no the perfect synonym for the word 'object' since it's sometimes used for goal(목표) or target(대상). So, It's difficult for translator to translate 'object': In philosophy, object is the same as '객체', in computer science, 오브젝트... and so on.

Examples: - 물건을 살 때는 한 번 더 생각해요. => Think one more time when you buy something. - 길에 떨어진 물건을 주우면 어떻게 해야 돼요? => What should you do if you find something on your way? - '앎'은 사물의 이치를 끝까지 탐구함에 의해서 얻어진다. (사물 can also mean "matters and events" together, as in here) => 'Knowledge' is gained by thoroughly exploring the principles governing matters and evens. - 사물 인터넷 => Internet of things. (a case of 사물 which is likely to be used a lot)