Hey Ksenia "Sobre" and "em cima" are synonimous. But the adverb "em cima" is very literal, so you should use it if you wanna describe things those are imediatelly above something. For example, "o livro está em cima da mesa." About "sobre", it works as a synonym of "em cima", but it`s most used as "about". For example, "esse livro é sobre pássaros - this book is about birds". 2) Which preposition should I use to say ''the picture is over// above the couch (=on the wall over//above the couch)? O quadro está .... o sofá. ***You should say "o quadro está acima do sofá 3) Which preposition should I use to say ''the picture is on the couch (= lies on the couch)? O quadro está .... o sofá. ***Here, it's better to say "o quadro está em cima do sofá". If you say "sobre", it's correct too, but it's not as usual as "em cima". I hope it helps you!

Just to complement the answer above, I guess the main diference between "em cima de" and "sobre" is that "sobre" is more formal. In casual conversation we use more "em cima de". Moreover, there is "acima", which means above ("em cima" = on). So you should say "O quadro está ACIMA do sofá" = The picture is above the couch. or "O quadro está em cima do sofá"/"O quadro está sobre o sofá" = The picture is on the couch.

1) no difference. 2) o quadro esta ACIMA do sofa. (do = de + o). I would not say "em cima" but "acima" in this case. the difference is subtle but exists. "em cima do sofa" = using the sofa as support. "acima do sofa" right over the sofa, on the wall. 3) o quadro esta NO sofa. (no = em + o).

sobre = em cima de / acima sob = embaixo de / abaixo abraços

1) They are almost interchangeable for static situations. However "sobre" is more dynamic and do not imply contact. "O avião voou sobre Londres". Everybody would lough if you say "O avião voou em cima de Londres" 2) O quadro está acima do sofá. 3) O quadro está em cima do sofá. 2) and 3) O quadro está por cima do sofá.