Paul se rend compte de la difficulté.

It is a fixed expression meaning to realize, to notice, or to become aware of something.

So:

Paul se rend compte de la difficulté.
= Paul realizes the difficulty.
= Paul becomes aware of the difficulty.

It is best learned as one unit: se rendre compte de + noun / clause.

The verb here is se rendre compte de, which is a pronominal expression. The se is part of the verb, not a separate word with a literal meaning like himself in English.

In this sentence:

• Paul se rend compte...
• literally, the reflexive pronoun matches the subject Paul

With other subjects, it changes:

• Je me rends compte
• Tu te rends compte
• Il/Elle se rend compte
• Nous nous rendons compte
• Vous vous rendez compte
• Ils/Elles se rendent compte

So you usually learn it as se rendre compte de rather than just rendre compte.

Because Paul is third person singular.

The verb is rendre, and in the present tense:

• je rends
• tu rends
• il/elle/on rend
• nous rendons
• vous rendez
• ils/elles rendent

Since Paul = he, you use il rend, so:

Paul se rend compte...

The expression is se rendre compte de quelque chose — to realize something / to become aware of something.

So de is required by the expression. It introduces the thing being realized.

Examples:

• Je me rends compte de mon erreur. = I realize my mistake.
• Elle se rend compte du problème. = She realizes the problem.
• Nous nous rendons compte de la situation. = We realize the situation.

You generally cannot drop the de here.

Because the full pattern is:

se rendre compte de + noun

So when the noun is la difficulté, it comes after de:

• de + la difficulté → de la difficulté

This is just the normal combination of the preposition de with the feminine singular article la.

Compare:

• de la difficulté = of the difficulty
• du problème = of the problem (de + le = du)
• des difficultés = of the difficulties (de + les = des)

French contracts de + le into du, and de + les into des.

But de + la does not contract.

So:

• de + le problème → du problème
• de + les difficultés → des difficultés
• de + la difficulté → de la difficulté

That is why the sentence keeps de la.

It is generally neutral and very common. You can use it in everyday conversation and in writing.

For example:

• Je me rends compte que j’ai oublié mon téléphone.
• Il se rend compte de son erreur.

It is one of the most common ways to say realize in French.

Yes. It can be followed by either:

• de + noun
• que + clause

Examples:

• Paul se rend compte de la difficulté.
  = Paul realizes the difficulty.

• Paul se rend compte que c’est difficile.
  = Paul realizes that it is difficult.

So if what follows is a thing or noun phrase, you use de.
If what follows is a full idea / statement, you often use que.

In this expression, compte is part of the fixed phrase se rendre compte. Historically, it is related to ideas like account or reckoning, but in modern French, learners should not try to translate it word by word.

So although compte can mean things like:

• account
• count
• bill (in some contexts)

in se rendre compte, the whole expression simply means to realize.

Not naturally. A very literal breakdown might look strange:

• Paul = Paul
• se rend compte de = realizes / becomes aware of
• la difficulté = the difficulty

A word-for-word translation would not sound idiomatic in English. The best approach is to translate the whole expression naturally:

Paul realizes the difficulty.
or
Paul becomes aware of the difficulty.

A simple pronunciation guide is:

Paul se rend compte de la difficulté
≈ Pol suh rahn kont duh la dee-fee-kul-tay

A few useful notes:

• rend sounds nasal: rahn
• compte is pronounced like kont
• the p in compte is silent
• difficulté ends with -té, pronounced tay

In connected speech, French sounds smoother than the spelling may suggest.

The singular is completely possible and natural if you mean the difficulty as one specific difficulty or the general difficulty of something.

So:

• la difficulté = the difficulty
• les difficultés = the difficulties

Both can work depending on meaning:

• Paul se rend compte de la difficulté. = Paul realizes the difficulty.
• Paul se rend compte des difficultés. = Paul realizes the difficulties.

The singular in your sentence is fine.