Paul pose la planche contre le mur.

Because pose is the 3rd person singular form of the verb poser in the present tense:

• je pose
• tu poses
• il / elle / on pose

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.

Here, poser means something like to place, to set down, or to put.

In this sentence, because of contre le mur, it suggests that Paul is placing / leaning the board against the wall.

So poser is a very common everyday verb for putting something somewhere.

Because planche is a feminine singular noun in French.

That means it takes the feminine singular definite article la:

• la planche = the board / the plank

French nouns have grammatical gender, so you have to learn the noun together with its article when possible.

Planche usually means board or plank.

Depending on context, it can refer to:

• a wooden board
• a plank
• sometimes a flat board-like object

In this sentence, board or plank is the most natural idea.

Contre means against.

So:

• la planche contre le mur = the board against the wall

It shows contact or support against a surface. That is why it fits naturally with the idea of leaning or placing something so that it rests on the wall.

Because contre is the preposition being used, and it is followed directly by the article:

• contre le mur = against the wall

You would not use au here because au means to the or at the and comes from à + le.

Compare:

• contre le mur = against the wall
• au mur = on the wall / to the wall, depending on context

So contre expresses the correct relationship here.

Because French contractions like au and du happen only with certain prepositions, especially à and de:

• à + le = au
• de + le = du

But contre does not contract with le, so it stays:

• contre le mur

No, it is very normal French word order:

• Paul = subject
• pose = verb
• la planche = direct object
• contre le mur = prepositional phrase showing location/position

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:

• placing the board against the wall
• setting it there
• leaning it there

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:

• Paul sounds roughly like pol
• pose sounds like poze without a strong final consonant
• planche ends with the sh sound
• mur has the French u, which does not exist exactly in English

There is no major required liaison here that changes the basic shape of the sentence.

You put ne ... pas around the verb:

Paul ne pose pas la planche contre le mur.

That means Paul is not placing the board against the wall.

In everyday spoken French, the ne is often dropped:

Paul pose pas la planche contre le mur.

But in standard written French, use ne ... pas.

There are several common ways:

1. Est-ce que Paul pose la planche contre le mur ?
   Very common and neutral.

2. Paul pose la planche contre le mur ?
   Common in spoken French, using intonation.

3. 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?