Paul peint sur le mur blanc.

Peindre is the infinitive form (to paint).
In Paul peint sur le mur blanc, peint is the present tense, 3rd person singular form of peindre (for il/elle/on or a name like Paul).

Conjugation (present):

• je peins
• tu peins
• il / elle / on peint
• nous peignons
• vous peignez
• ils / elles peignent

In this sentence, peint is the present tense of peindre: Paul peint = Paul paints / Paul is painting.

But peint is also the past participle of peindre (as in Il a peint le mur = He painted the wall).
So the form is the same, but here it is clearly a verb in the present tense because it stands alone after the subject Paul, not after an auxiliary verb like a or avait.

Peint is pronounced like [pɛ̃], roughly like "pan" in "pan" but with a nasal vowel and no final n sound.
The final t is silent in normal speech.

You do not pronounce the n either; it just nasalizes the vowel e.
So: one syllable, no t, no n at the end.

Sur usually means on / on top of / onto a surface, so sur le mur = on the wall (on its surface).

• sur = on (a surface): écrire sur le tableau (to write on the board)
• dans = in, inside: dans la boîte (in the box)

Since a wall is a surface you paint on, sur is the normal preposition here.

Context decides the best English translation.

• on the wall: focusing on where the paint is: Paul peint sur le mur blanc
• onto the wall: if you want to stress the movement toward the wall

French sur can cover both on and onto depending on context; you don’t need to change the French sentence.

Le is the definite article (the), un is the indefinite article (a).

• le mur blanc = the white wall (a specific wall the speakers have in mind)
• un mur blanc = a white wall (one white wall, not previously identified)

So le mur blanc suggests Paul is painting on a particular, known white wall.

Most French adjectives come after the noun: un mur blanc, un livre intéressant, une voiture rouge.

Only certain categories of adjectives (often short and very common ones like beau, petit, grand, jeune, vieux, bon, mauvais, joli, gros) typically go before the noun.
Blanc (white) normally comes after the noun, so un mur blanc is the standard order. Un blanc mur is incorrect.

Adjectives in French agree in gender and number with the noun they describe.

• mur (wall) is masculine singular: le mur
• the masculine singular form of the adjective is blanc
• the feminine singular form would be blanche (for a feminine noun: une maison blanche)

So we use blanc because mur is masculine. You just have to learn the gender of nouns (e.g., le mur, la maison).

By default, peindre sur le mur suggests he is painting on the surface of the wall (for example, drawing a picture or graffiti), not necessarily repainting the wall itself one solid color.

If you want to say he is painting the wall (as a job, covering it), French usually says:

• Paul peint le mur en blanc. = Paul is painting the wall white (changing its color).

So:

• peindre sur le mur = paint on the wall’s surface
• peindre le mur (often with a color) = paint the wall itself

French often uses a special expression instead of a separate continuous tense. You can say:

• Paul est en train de peindre sur le mur blanc.

Être en train de + infinitive emphasizes the ongoing nature of the action, similar to English is doing X right now.

In standard written French, you would not normally say Paul il peint…; it’s redundant (subject noun + subject pronoun).
The correct neutral version is: Paul peint sur le mur blanc.

However, in informal spoken French, you sometimes hear sentences like Paul, il peint sur le mur blanc, often with a pause or comma. It sounds conversational, a bit like “Paul, he paints on the white wall.” in English.

Yes, Sur le mur blanc, Paul peint is grammatically correct.
Putting sur le mur blanc at the beginning adds emphasis to the location, like saying:

• On the white wall, Paul is painting.

It sounds a bit more literary or stylistic than the neutral order Paul peint sur le mur blanc.

You need to make mur and blanc plural, and use the plural definite article:

• Paul peint sur les murs blancs. = Paul paints on the white walls.

Changes:

• le mur → les murs
• blanc → blancs (add -s to agree with plural murs)

No. Mur (wall) is always spelled mur in modern French.

Mûre (with a circumflex, feminine) is a different word:

• une mûre = a blackberry (the fruit)
• mûr / mûre (adjective) = ripe, or figuratively mature

So le mur blanc (the white wall) is unrelated to une mûre (a blackberry).