Portuguese has, as most other languages, something like 200 prepositions. However most of them are complex expressions formed by several words.
a partir de... => from...
a pedido de... => at the request of...
ao lado de ... => beside...
arredor de ... => around..
à procura de... => in search of..
If the meaning of the single components is known, partir => to leave a place, pedido => request, lado => side it is possible to guess the meaning. Compound prepositions are in general semantically motivated. The real problem are the prepositions that are not semantically montivated. A painting can hang on the wall, in the wall or at the wall and in some languages it hangs on the wall, in others in the wall and in others at the wall.
Portuguese: (in = em)
O quadro pendura na parede.
Spanish: (in = en)
El cuadro está colgado en la pared.
French: (at = à)
Le tableau est accroché au mur.
Italian: (at = a)
Il quadro é appeso alla parete.
English: ( on)
The painting hangs on the wall.
Das Bild hängt an der Wand.
Persian: (on = bar rouye)
عکس تابلو بر روی دیوار آویزان است
aks bar rouye diwar aawizan ast.
French and Spanish use a preposition that normally has a completely different meaning. The preposition a / à has a lot of meanings however it rarely describes, this example is an exception, the position in space of an object relative to another object. Normally it describes a mouvement. (Je vais à Paris. / Voy a Paris => I go to Paris.)
Perhaps Persian and English are the most comprehensible, both of them use on and on is used as well if an object lies horizontally over another object. (The book lies on the table.)
The basic problem is that we have a much broader range of possible relationships between object than between main clauses and subordinate clauses. The relationships between a main clause and subordinate clause is described by conjunctions, see 19, and the type of possible relationships is limited. There are causal, condicional, concessive, modal, final and adversative relationships. Normally to a conjunctions in one language there is a corresponding one in the other language that fits in any context, in other words conjunction x corresponds to conjunction x and any time we use x in one language we can use y in the other language as well.
The case of preopsitions is more complex and the relationships between objects are often vague. If one object is placed vertically on another object we have a clear relationship and for these easy to see relationships we have a preposition in any language and we can find the right preposition in any dictionnary. Same thing is true if an object is vertically under another object. However in real live things are more complicated. An object can be placed higher / lower than another one, but not vertically. If a castle is placed on a hill above the city, it is not placed vertically on the city, and the Dead Sea is below sea level, but not under the ocean. Some languages make a distinction between on / above and under / below, others not.
Prepositions have always a basic meaning, that we can find in any dictionnary. If the relationship is described by this basic meaning, we have a corresponding preposition: sobre = on. O livro esta sobre a mesa <=> The book is on the table. But if the relationship is unclear, a painting can hang in the wall, on the wall or at the wall it is well possible that in the foreign language a completely different preposition is used. If the Portuguese native speakers find, to give an example, that a painting hangs in the wall and the English native speakers find that it hangs on the wall, they won't use the same prepositions. If the Portuguese native speakers find that one talks ~of somebody, Falam dela and the English native speakers talks about somebody, They talk about her, they won't use the same preposition. This author would say that most problems in the use of prepositions arise because the relationships to be described are vague.
We will discuss in this chapter the critical aspects of the most used prepositions. Their use is critical if they describe a relationship that is more or less far away from their basic meaning, what happens for instance if the relationship can be interpreted in different ways. In this case it happens very often that we translate in English with a preposition whose basic meaning is completely different.
Prepositions are actually not a grammar problem, but a lexical problem. If we have a set of rules that applies to an infinite number of situations, we have grammar rules. If we know how to use the tenses and mood in conditional clauses we can contruct an indefinite number of conditional clauses. However if we know that the ball is in the box, we can't deduce from that the apples hang in the tree, what is actually true, although they hang on the tree. Prepositions are a lexical problem. To ask why a certain preposition is used in a certain context is as useful as asking why the table is called a table.
This chapter can sensitize the reader for certain phenomenon and that can be helpful to memorize the contexts in which they are used. However learning all the posssible contexts by heard is a little bit boring. It is better to read and hear the novels included in this website. That way most of them will be learned automatically.