Because pose is the 3rd person singular form of the verb poser in the present tense:
Since Paul is he, French uses pose.
This often feels strange to English speakers because English usually adds -s in the third person singular, but French -er verbs do not work that way in spelling.
Planche usually means board or plank.
Depending on context, it can refer to:
In this sentence, board or plank is the most natural idea.
Because contre is the preposition being used, and it is followed directly by the article:
You would not use au here because au means to the or at the and comes from à + le.
Compare:
So contre expresses the correct relationship here.
Because French contractions like au and du happen only with certain prepositions, especially à and de:
But contre does not contract with le, so it stays:
No, it is very normal French word order:
So the structure is basically:
Subject + Verb + Object + Location/Position
That is quite similar to English.
Yes. Both are reasonable ways to understand it.
French poser is broad enough that, in context, it can describe:
The exact English translation depends on the situation, but the French sentence itself is perfectly natural.
A careful pronunciation would be approximately:
Paul pose la planche contre le mur
pol poz la plɑ̃ʃ kɔ̃trə lə myr
A few helpful points:
There is no major required liaison here that changes the basic shape of the sentence.
There are several common ways:
Est-ce que Paul pose la planche contre le mur ?
Very common and neutral.
Paul pose la planche contre le mur ?
Common in spoken French, using intonation.
Paul pose-t-il la planche contre le mur ?
More formal, with inversion.
All three mean something like Is Paul placing the board against the wall?