Le secouriste aide la victime.

They are definite articles, the French equivalent of the.

• le secouriste = the rescuer / the first-aider
• la victime = the victim

French articles change depending on the gender of the noun:

• le = masculine singular
• la = feminine singular
• les = plural

So here, secouriste is treated as masculine in this sentence, and victime is feminine.

Because aide is a conjugated verb, while aider is the infinitive.

The verb is aider = to help.

In this sentence, the subject is le secouriste, which is he/she/it in grammar terms, so French uses the 3rd person singular present form:

• j’aide = I help
• tu aides = you help
• il/elle aide = he/she helps

So Le secouriste aide la victime uses aide because it means helps.

Yes. This sentence follows the very common pattern:

subject + verb + object

• Le secouriste = subject
• aide = verb
• la victime = object

So it works just like:

The rescuer helps the victim.

This is one reason the sentence feels straightforward to English speakers.

Not necessarily. Secouriste can refer to either a male or female rescuer; the article often shows the gender in context.

For example:

• le secouriste = the male rescuer / the rescuer (masculine)
• la secouriste = the female rescuer

This is common with some profession or role nouns in French: the noun itself may stay the same, while the article changes.

Yes. Victime is a grammatically feminine noun, so it takes la, even if the victim is male.

That is an important idea in French: grammatical gender is not always the same as biological sex.

So you can say:

• la victime for a woman
• la victime for a man

The noun stays feminine because that is just its grammatical gender.

A simple pronunciation guide is:

luh suh-koo-reest ed lah veek-teem

A bit more carefully:

• Le ≈ luh
• secouriste ≈ suh-koo-reest
• aide ≈ ed
• la ≈ lah
• victime ≈ veek-teem

A few useful notes:

• The final -e in aide is not pronounced separately.
• The final -e in victime is also not pronounced.
• French r is not like the English r; it is made farther back in the throat.
• ai in aide sounds like eh/ay, but in this word it is best thought of simply as ed.

Usually, no important liaison is made here.

You would normally say:

• Le secouriste without liaison between le and secouriste
• aide la without liaison
• la victime without liaison

So this sentence is fairly easy to pronounce word by word.

Because some French nouns have the same form for masculine and feminine. In those cases, the article tells you the gender:

• un secouriste
• une secouriste

or

• le secouriste
• la secouriste

So the noun itself does not always change.

Yes. The base form is aider, and it is a regular -er verb, which is one of the most common verb types in French.

Its present-tense forms are:

• j’aide
• tu aides
• il/elle/on aide
• nous aidons
• vous aidez
• ils/elles aident

So aide in the sentence is a very normal, predictable form.

You change the articles, the noun if needed, and the verb form.

Examples:

• Les secouristes aident la victime. = The rescuers help the victim.
• Le secouriste aide les victimes. = The rescuer helps the victims.
• Les secouristes aident les victimes. = The rescuers help the victims.

Notice:

• le / la becomes les
• secouriste becomes secouristes in writing
• victime becomes victimes in writing
• aide becomes aident with a plural subject

Also, in speech, the -ent in aident is usually not pronounced.

Yes. That would change the to a/an.

For example:

• Un secouriste aide une victime. = A rescuer helps a victim.

So:

• le / la = the
• un / une = a / an

That is a very common substitution learners practice early on.