Les premiers secours sont nécessaires après l’arrêt cardiaque.

Because premiers secours is a fixed French expression meaning first aid. It is normally used in the plural.

So French treats it as a set of emergency actions rather than one single help. That is why you say:

• les premiers secours
• not usually le premier secours

This is similar to how some ideas are naturally plural in one language but not in another.

Because it agrees with secours.

In les premiers secours:

• les = plural
• secours = plural noun
• premiers = plural adjective

So premier becomes premiers to match the noun.

Because the subject is plural: les premiers secours.

In French, the verb must agree with the subject:

• singular: Le secours est nécessaire
• plural: Les premiers secours sont nécessaires

The adjective after the verb also agrees with the subject:

• singular masculine: nécessaire
• plural masculine: nécessaires

So sont nécessaires matches les premiers secours.

It is agreeing with les premiers secours, which is plural.

French adjectives often change form to match the noun they describe. Here the adjective is used after être and still agrees with the subject:

• masculine singular: nécessaire
• masculine/feminine singular: nécessaire
• plural: nécessaires

So the final s marks the plural.

Because arrêt begins with a vowel sound, so le becomes l’ before it.

This is called elision:

• le arrêt → not correct
• l’arrêt → correct

French often does this with le, la, je, ne, etc. before a vowel or mute h.

Here arrêt means stoppage or arrest, so arrêt cardiaque means cardiac arrest.

The circumflex in arrêt is part of the standard spelling. In many words, the circumflex reflects an older historical spelling, often where an s used to exist in older French or related forms. For a learner, the main thing is simply to memorize arrêt with ê.

Also note that arrêter is the related verb meaning to stop.

Because cardiaque is an adjective meaning cardiac or related to the heart, and in French many descriptive adjectives come after the noun.

So:

• arrêt = arrest/stoppage
• cardiaque = cardiac

Together: arrêt cardiaque

This is the normal French order for this expression.

Because après is a preposition, and it is followed directly by a noun phrase.

So you say:

• après l’arrêt cardiaque = after the cardiac arrest

You do not add de after après in this structure.

Compare:

• après le repas = after the meal
• après l’accident = after the accident

Yes, and the meaning changes slightly.

• après l’arrêt cardiaque can sound more specific or more general in a medical/contextual way, depending on the situation.
• après un arrêt cardiaque means after a cardiac arrest, often presenting it as one instance among possible cases.

In medical or general statements, French often uses the definite article where English might prefer no article or an indefinite one. So après l’arrêt cardiaque sounds very natural.

Because article usage is different in French.

French very often uses an article where English uses none. So even if English says simply first aid, French normally says:

• les premiers secours

This is just a normal difference between the two languages and is something learners have to get used to.

In this sentence, secours is plural, even though its written form looks the same as the singular.

Some French nouns ending in -s do not visibly change in the plural. The article and agreement tell you the number:

• le secours = the help / the rescue
• les secours = the emergency services / aid / rescue efforts

Here, les premiers secours sont nécessaires clearly shows plural because of:

• les
• premiers
• sont
• nécessaires

A careful pronunciation is roughly:

Lay pruh-myay su-koor son nay-say-sair ah-pray la-ray kar-dyak

A few useful points:

• les sounds like lay
• premiers sounds roughly like pruh-myay
• secours sounds like su-koor
• sont is nasal, roughly son
• après sounds like ah-pray
• l’arrêt flows together smoothly because of the elision
• cardiaque sounds roughly like kar-dyak

There is no strong English-style stress on one word; French rhythm is smoother and more even.

The structure is:

Subject + verb + adjective + time phrase

So:

• Les premiers secours = subject
• sont = verb
• nécessaires = adjective describing the subject
• après l’arrêt cardiaque = prepositional phrase giving the time/context

A literal breakdown would be:

The first aid measures are necessary after the cardiac arrest.

That is why the sentence is straightforward grammatically once you identify the plural subject.