在桌子上-on the (upper surface of the) table; *Here* '上' is different from 'above/over'. 在墙上-on the (surface of the) wall. When you note something down in your notebook, we say '写/记录 在笔记本上' or '在笔记本上 写/记录'. Because we write/scribe/paint/draw on the *upper *surface* of the table/wall/notebook, so here '上' fits in. Or you can rephrase '在桌子上' as '在桌子的上面'. And you'd better take '在...上' as a set phrase.

在。。。上 is a phrase, means on... , like 在书上 on the book, 在桌子上 on the table/desk. Liz.

1. 在...上是介词， 2. 上车，上当，上学， 动词 3. 桌上 ，名词

老师看见我在桌子上写东西 the teacher saw me writing something on the table 在。。。上 means on 上表示方位 在是动词 和on是一个意思 在。。。下面 在。。。中间 on top of on bottom of in the middle of something like that