Le contrôleur technique décèle l’usure de l’amortisseur.

Because décèle is the conjugated form of the verb déceler.

In this sentence, the subject is Le contrôleur technique (the technical inspector), which is third person singular, so in the present tense you use:

• je décèle
• tu décèles
• il / elle / on décèle

So Le contrôleur technique décèle... means the inspector detects / spots...

This is a spelling change used to keep the pronunciation natural.

The infinitive is déceler, but in some present-tense forms the e in the stem changes to è:

• je décèle
• tu décèles
• il décèle
• ils décèlent

But:

• nous décelons
• vous décelez

This kind of change is common in French verbs where the pronunciation would otherwise be awkward or different.

Because both usure and amortisseur begin with a vowel sound.

In French, le or la becomes l’ before a vowel or a silent h. This is called elision.

So:

• la usure → l’usure
• le amortisseur → l’amortisseur

This makes the sentence easier to pronounce.

Because the noun usure is followed by de to show what is worn: the wear of the shock absorber.

You start with:

• de + l’amortisseur

Since amortisseur begins with a vowel, the article is already l’, so there is no further contraction.

Compare:

• de + le moteur → du moteur
• de + l’amortisseur → de l’amortisseur

So de l’amortisseur is the correct form.

Amortisseur is masculine.

That is why the full article is le, which becomes l’ before the vowel:

• le amortisseur → l’amortisseur

If you look it up in a dictionary, you will normally see it marked as masculine.

It is the direct object of the verb décèle.

Structure:

• Le contrôleur technique = subject
• décèle = verb
• l’usure de l’amortisseur = direct object

Inside that object:

• l’usure = the main noun
• de l’amortisseur = a complement that specifies what is worn

So literally it is something like the wear of the shock absorber.

Not really. Contrôleur often means someone who checks, inspects, or monitors something, not necessarily a controller in the English sense.

In this context, contrôleur technique is better understood as something like:

• technical inspector
• vehicle inspector
• inspection officer

So this is a good word to treat carefully, because contrôleur is not always a direct match for English controller.

Yes, in many contexts détecte would be understandable, but décèle has a slightly more specific nuance.

• détecter = to detect
• déceler = to detect, uncover, discern, spot signs of

So décèle can sound a little more like noticing evidence of something, rather than simply registering its presence.

In a technical or inspection context, décèle works very well.

A careful pronunciation is approximately:

Le contrôleur technique décèle l’usure de l’amortisseur.
IPA: lə kɔ̃.tʁɔ.lœʁ tɛk.nik de.sɛl ly.zyʁ də la.mɔʁ.ti.sœʁ

A rough English-style guide would be:

luh kon-TRO-lur tek-NEEK day-SELL loo-ZOOR duh la-mor-tee-SUR

A few points:

• contrôleur has a nasal on sound in con
• décèle sounds like day-sell
• usure begins with a u sound that French learners often need practice with
• amortisseur ends with the French -eur sound, not an English -er

It is in the present tense: the French présent de l’indicatif.

So décèle means:

• detects
• is detecting

depending on context

In a sentence like this, the simple present in English is usually the best match: The technical inspector detects...

French often expresses this idea with de, where English may prefer of or on depending on the noun.

So:

• l’usure de l’amortisseur
  literally: the wear of the shock absorber

But in natural English, that may become:

• the wear on the shock absorber

This is a good example of how French and English organize noun phrases differently. French uses de very often in places where English uses several different prepositions.

As written, it most naturally refers to a specific inspector or the inspector in a particular context, because of the definite article Le.

• Le contrôleur technique... = The technical inspector...

If you wanted to talk about technical inspectors in general, French would usually phrase it differently, for example with a plural or a more general construction.

So this sentence sounds like it is describing what the inspector does in a given situation.