Elle a eu un frisson en entrant dans la grotte humide.

A eu is the passé composé of avoir.

It is used here because the sentence describes a single completed event: she suddenly felt a shiver at that moment. French often uses the passé composé for this kind of one-time action or reaction.

If you used avait, that would be the imparfait, which would suggest more of an ongoing state or background description, not a sudden reaction.

Because the verb uses avoir as its auxiliary, the past participle usually does not agree with the subject.

So:

• elle a eu
• il a eu

Both use eu.

Agreement with a past participle after avoir only happens when there is a direct object before the verb, which is not the case here.

Yes. Avoir un frisson is a very natural French expression.

French often uses avoir + noun where English might use a verb or a different structure. So instead of building the idea around a verb like to shiver, French can say have a shiver.

A frisson can be:

• a shiver from cold
• a chill
• a shudder from fear or emotion

So the expression is very normal and idiomatic.

Because frisson is being treated as one countable shiver/chill.

So un frisson means:

• one shiver
• a sudden chill
• a shudder

French often uses the indefinite article in this kind of expression:

• avoir un doute
• avoir une idée
• avoir un frisson

Leaving out un would be ungrammatical here.

En entrant is the structure en + present participle. In French grammar, this is often called the gérondif.

It usually expresses something like:

• while entering
• on entering
• upon entering
• sometimes by entering, depending on context

Here it gives the circumstance in which the shiver happened: it happened as she entered the cave.

Because this is not a past participle. It is a present participle used after en.

• entrant = present participle of entrer
• entrée = past participle, or sometimes an adjective/noun depending on context

After en, French uses the -ant form:

• en entrant
• en parlant
• en sortant

This form does not agree in gender or number, so it stays entrant even though the subject is elle.

The understood subject of en entrant is the same as the subject of the main verb.

So in this sentence:

• Elle a eu un frisson
• en entrant dans la grotte humide

The person entering is also elle.

This is an important rule in French: with en + present participle, the subject is normally the same as the subject of the main clause.

Yes, you could say something like quand elle est entrée dans la grotte humide.

That would be perfectly understandable, but it is a little heavier and more explicit.

Using en entrant is:

• more compact
• very natural
• slightly smoother stylistically

So both are possible, but en entrant is an elegant and common way to express as she was entering / upon entering.

Because dans is the normal preposition for movement into something.

With entrer, French commonly uses:

• entrer dans une maison
• entrer dans une pièce
• entrer dans une grotte

À la grotte would mean something more like at the cave, not into the cave.

En la grotte is not correct French. Also, en is not the normal choice here with a specific noun like la grotte.

Because in French, many descriptive adjectives are placed after the noun.

So:

• une grotte humide
• une pièce sombre
• un mur froid

Humide is a normal adjective that usually comes after the noun it describes.

Here humide clearly modifies grotte, so it means the cave is damp or humid.

Yes. Grotte is a feminine noun, so it takes la in the singular:

• la grotte
• une grotte

That is also why, if an adjective had a different feminine form, it would need to match. In this case, humide happens to have the same form in masculine and feminine singular, so you do not see a visible change.

So:

• un lieu humide
• une grotte humide

The adjective agrees in gender, but its written form stays the same here.