Ma sœur garde toujours une petite boussole dans sa poche quand elle passe la nuit au camping.

Here garde means keeps or carries/has on her.

So Ma sœur garde toujours une petite boussole dans sa poche means that she regularly keeps a small compass in her pocket.

The verb is garder, which often means:

• to keep
• to look after
• to guard

In this sentence, keep is the best match.

Because sœur is a feminine noun, so the possessive adjective must also use the feminine form:

• mon = my + masculine singular noun
• ma = my + feminine singular noun
• mes = my + plural noun

So:

• mon frère = my brother
• ma sœur = my sister
• mes parents = my parents

A useful extra point: before a feminine noun starting with a vowel sound, French often uses mon instead of ma for ease of pronunciation:

• mon amie = my female friend

But sœur starts with a consonant sound, so ma sœur is correct.

They are both in the present tense, with a third-person singular subject.

• Ma sœur garde = my sister keeps
• elle passe = she spends

For regular -er verbs in the present tense, the il/elle/on form usually ends in -e:

• je garde
• tu gardes

• il/elle garde

• je passe
• tu passes
• il/elle passe

So the ending matches the subject:

• ma sœur → third person singular
• elle → third person singular

In French, short adverbs like toujours often go after the conjugated verb.

So:

• Elle garde toujours une petite boussole
• literally: she keeps always a small compass
• natural English: she always keeps a small compass

This is very common in French:

• Il mange souvent ici = He often eats here
• Nous arrivons toujours tôt = We always arrive early

So garde toujours is normal French word order.

Because boussole is a feminine singular noun.

That affects both the article and the adjective:

• une = feminine singular a
• petite = feminine singular form of petit

So:

• un petit livre = a small book
• une petite boussole = a small compass

Also, petit/petite is one of the common adjectives that often comes before the noun in French.

Because poche is a feminine singular noun, and the possessive adjective agrees with the thing possessed, not with the owner.

So:

• son = his/her + masculine singular noun
• sa = his/her + feminine singular noun
• ses = his/her + plural noun

That means:

• sa poche = his pocket / her pocket
• son sac = his bag / her bag

Even though the owner is she, that is not the reason for sa. The reason is that poche is feminine.

Because French normally requires an explicit subject in each clause.

The sentence has two clauses:

• Ma sœur garde toujours une petite boussole dans sa poche
• quand elle passe la nuit au camping

In English, we also usually repeat the subject:

• My sister always keeps a small compass in her pocket when she spends the night camping.

French does not usually drop subject pronouns the way some other languages do, so elle is necessary.

Passer la nuit is an idiomatic expression meaning to spend the night.

So:

• elle passe la nuit au camping = she spends the night at the campsite / while camping

It is not usually translated word-for-word as passes the night in natural English, even though that is the literal structure.

You can see the same pattern in other expressions:

• passer la journée = to spend the day
• passer la soirée = to spend the evening
• passer le week-end = to spend the weekend

French commonly uses the definite article in expressions of time like this:

• passer la nuit
• passer la journée
• passer la matinée

English often says:

• spend the night
• spend the day

So the article is just part of the normal French expression. It is not optional here.

Because à + le contracts to au in French.

So:

• à + le = au
• à + les = aux

Examples:

• au camping = at the campsite / at the campground
• au parc = at the park
• aux États-Unis = in the United States

So elle passe la nuit au camping is the correct contracted form.

Not exactly.

In French, un camping usually means a campsite, campground, or holiday camping site.

So:

• au camping = at the campsite / at the campground

It does not usually mean the activity itself in exactly the same way English camping does.

Depending on context, the sentence could be understood as:

• when she spends the night at the campsite
• when she spends the night camping

The present tense is used because the sentence describes a habitual action.

It means something like:

• My sister always keeps a small compass in her pocket when she spends the night camping.
• My sister always keeps a small compass in her pocket whenever she stays overnight at a campsite.

French often uses the present tense for things that happen regularly or generally, just like English does with always and when/whenever.