Au fur et à mesure que mars avance, le jardin devient plus vert.

Au fur et à mesure que is a fixed expression that means something like as, as... goes on, or gradually as.

In this sentence, it introduces a process that happens little by little over time:

• Au fur et à mesure que mars avance = As March goes on / As March progresses

A helpful way to understand it is that it shows two things developing together:

• March moves forward
• the garden gets greener

So it is not just when, but more in step with the progress of something.

Because que introduces a full clause.

Here, the clause is:

• mars avance

So the structure is:

• au fur et à mesure que + clause

Examples:

• Au fur et à mesure que le temps passe...
• Au fur et à mesure que les enfants grandissent...

If you are following it with a noun instead of a full clause, French often uses a different structure, such as au fur et à mesure de or au fur et à mesure des... in some contexts.

Literally, mars avance means March moves forward.

But in natural English, you would usually translate it more idiomatically as:

• March goes on
• March progresses
• March advances

French often uses verbs like avancer with time periods in a slightly more vivid way than English does. It personifies the month a little, but this sounds normal in French.

In French, names of months are normally not capitalized.

So you write:

• mars
• avril
• mai

This is different from English, where we write March, April, May.

The only time you would capitalize mars is if it appears at the beginning of a sentence, or if it were a proper noun in another sense.

French often uses the present tense to describe:

• a general truth
• a recurring situation
• something unfolding in a vivid, immediate way

Here, the sentence describes a typical seasonal process, so the present tense is very natural:

• mars avance
• le jardin devient plus vert

English might also use the present in a similar sentence:

• As March goes on, the garden becomes greener.

So this is not necessarily talking about one exact moment right now; it can express a general pattern.

Because devenir means to become, and it emphasizes change.

• Le jardin devient plus vert = The garden is becoming greener / The garden becomes greener
• Le jardin est plus vert = The garden is greener

So:

• est describes a state
• devient describes a transition into that state

Since the sentence is about the garden changing as March progresses, devient is the better choice.

Because vert is an adjective here, and plus is modifying that adjective.

• plus vert = greener / more green

This is the normal comparative structure with adjectives in French:

• plus grand = taller
• plus froid = colder
• plus vert = greener

Plus de is used with nouns, not adjectives:

• plus de fleurs = more flowers

So here, because vert describes the garden, plus vert is correct.

Yes, le jardin literally means the garden.

French uses definite articles very often, sometimes more often than English learners expect. In this sentence, it probably refers to:

• a specific garden already understood from context, or
• a garden being spoken about in a general but definite way

French would not normally drop the article here. Saying just jardin devient plus vert would be incorrect.

It is generally neutral and very common in both writing and speech.

It sounds a bit more structured than simply quand or comme, because it emphasizes gradual progression. But it is not overly formal.

For example:

• Au fur et à mesure que l’année avance...
• Au fur et à mesure que je comprends mieux...

So it is a very useful expression to learn.

Not by itself. The correct expression would be de plus en plus vert, which means greener and greener or more and more green.

So you could say:

• Au fur et à mesure que mars avance, le jardin devient de plus en plus vert.

That would sound very natural too, and it would put even more emphasis on the gradual change.

Compare:

• plus vert = greener
• de plus en plus vert = greener and greener

A few things may stand out for English speakers:

• au sounds like o
• fur does not sound like English fur
• mesure has the French u sound and a soft j sound in the middle
• que is usually pronounced like kuh or a very light kə, depending on speed

A rough pronunciation guide is:

• oh fur ay ah muh-zur kuh

But it is better to learn it as one chunk, because native speakers often say it smoothly as a fixed expression.

A little, yes.

The phrase mars avance treats March as if it can move forward. That is a mild kind of personification, but in French it is completely normal and not especially poetic.

English can do something similar:

• As March moves along...
• As the month goes on...

So it is slightly figurative, but very ordinary.

Yes. You could also say:

• Le jardin devient plus vert au fur et à mesure que mars avance.

That means the same thing.

The original version puts the time/process part first:

• Au fur et à mesure que mars avance...

This creates a nice flow and sets up the gradual change before giving the result.

So both are correct, and the choice is mostly about style and emphasis.