Je vais essuyer la mousse sur le sol avant que Marie arrive.

Je vais + infinitive is the near future. It is very common in everyday French and often suggests that the action is planned or will happen soon.

So:

• je vais essuyer = I’m going to wipe
• j’essuierai = I will wipe

Both are possible, but je vais essuyer often sounds more natural in conversation.

Essuyer basically means to wipe.

Depending on context, English may use different verbs:

• wipe
• wipe up
• dry
• sometimes wipe off

Here it means wiping the foam away. French often uses one simple verb where English prefers a phrasal verb like wipe up.

Here, la mousse means foam, froth, or suds, depending on the situation.

That word can also mean other things in other contexts, such as:

• mousse as a dessert
• moss

So learners often need to rely on context. In a cleaning-type sentence, foam/suds is the natural meaning.

Yes, this is a very natural question.

Sur le sol tells you where the foam is: it is on the floor.

But if you want to emphasize removing it from the floor, many speakers might prefer something like:

• essuyer la mousse du sol
• or another phrasing such as nettoyer la mousse sur le sol

So sur le sol is understandable, but du sol can sound more natural if the idea is specifically wiping it off the floor.

Use avant de + infinitive when the subject stays the same.

Example:

• Je vais essuyer la mousse avant de partir. The person wiping and the person leaving are the same.

But here, the subjects are different:

• je am wiping
• Marie is arriving

So French needs a full clause:

• avant que Marie arrive

That is why avant que is used.

After avant que, French normally uses the subjunctive.

So the structure is:

• avant que + subject + subjunctive verb

That is why you get:

• avant que Marie arrive

A useful thing to know: with arriver, the third-person singular subjunctive is arrive, which looks exactly like the present indicative form. So even though the form looks familiar, the grammar after avant que is still the subjunctive.

Sometimes, yes.

In more careful or formal French, you may see:

• avant que Marie n’arrive

This ne is called an expletive ne. It does not make the sentence negative.

So:

• avant que Marie arrive
• avant que Marie n’arrive

Both can be correct. The version without ne is very common in everyday French.

They are close, but not exactly identical.

• le sol = the floor / the ground, a noun
• par terre = on the floor / on the ground, a fixed expression

Le sol sounds a bit more neutral or formal.
Par terre is very common in everyday speech.

So a learner may often hear:

• Il y a de la mousse par terre.

But in your sentence, sur le sol is a more literal, slightly more formal way to express location.

Because after que, French keeps normal clause order:

• que + subject + verb

So:

• avant que Marie arrive

There is no inversion here. French does not usually change the word order after que in this kind of sentence.