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3)There is a peculiar syntactic structure in Chinese named locative/positional phrases 方位短语, which consists of a noun + a localizer (上,下,左,右,前,后,东,西,南,北,中,里,内,外;and some other localizers composed by one or two of these words, for instance, 东南), for instance, 桌子上,屋外,海上.
Chinese locative phrases can be used by theirselves, (×在)河那边是一大片金黄色的油菜花,煞是好看||| (到)屋里坐(吧),(到)屋里坐(吧)!and used with prepositions as well. When they are used together, the prep. often can be omitted. ------ (我该把_/你想让我把_)书放在哪里?------- 放(在)桌子上吧!
I think your confusion about when to use and when not to use a prep. arises on the occasion where a locative phrase is used. This is an complex problem. I'm sorry to say I havn't did close study on it, so I cannot rashly give a specious answer, though I have some vague ideas about it . Maybe you can find some information over this problem on the internet or some special reference materials.