En mars, les jours deviennent plus longs, et nous avons envie de marcher après le travail.

En mars is the normal French way to say in March.

• en is used with most months:
  
  • en mars = in March
  • en avril = in April
• dans usually means in with a sense of inside or in ... time:
  • dans la maison = in the house
  • dans trois jours = in three days
• au mars is not correct for months.

You may also hear au mois de mars, which also means in the month of March, but it is longer and more formal than en mars.

In French, general statements often use the definite article where English might not.

So:

• les jours deviennent plus longs = the days are becoming longer / days are becoming longer

French usually prefers les here because it is talking about days in general, especially the days in that time of year.

This is very common:

• Les enfants aiment jouer. = Children like to play.
• Les chats dorment beaucoup. = Cats sleep a lot.

English often drops the article; French often keeps it.

Deviennent comes from the verb devenir, which means to become.

So:

• les jours deviennent plus longs = the days are becoming longer

This suggests a change is happening.

If you said:

• les jours sont plus longs

that would mean the days are longer, describing the situation more statically.

So the difference is:

• deviennent = are becoming, are getting
• sont = are

In this sentence, deviennent is a good choice because in March the days are gradually getting longer.

Deviennent is the 3rd person plural present tense of devenir.

The subject is les jours, which is plural, so the verb must also be plural.

Present tense of devenir:

• je deviens
• tu deviens
• il/elle/on devient
• nous devenons
• vous devenez
• ils/elles deviennent

So:

• les jours deviennent = the days become / are becoming

Notice that devenir is an irregular verb.

Because longs agrees with les jours.

• jours is masculine plural
• so the adjective must also be masculine plural
• long → longs

That gives:

• les jours deviennent plus longs

Agreement examples:

• le jour est long = the day is long
• les jours sont longs = the days are long
• la nuit est longue = the night is long
• les nuits sont longues = the nights are long

Also, plus longs here means longer, as part of a comparison.

Here, plus is part of the comparative structure:

• plus + adjective = more + adjective

So:

• plus longs literally = more long
• natural English = longer

Other examples:

• plus grand = bigger / taller
• plus difficile = more difficult
• plus intéressant = more interesting

So les jours deviennent plus longs means the days are getting longer.

Avoir envie de is a very common French expression meaning:

• to feel like
• to want to
• to have the desire to

So:

• nous avons envie de marcher = we feel like walking / we want to walk

Literally, it is something like we have desire to walk, but in natural English you would not translate it word for word.

Very common patterns:

• avoir envie de + noun
  • J’ai envie d’un café. = I feel like a coffee.
• avoir envie de + infinitive
  • J’ai envie de dormir. = I feel like sleeping.

Because the expression is avoir envie de + infinitive.

So the pattern is fixed:

• avoir envie de faire quelque chose = to feel like doing something

Examples:

• J’ai envie de lire. = I feel like reading.
• Nous avons envie de sortir. = We feel like going out.
• Elle a envie de marcher. = She feels like walking.

You cannot normally say avoir envie marcher without de. The de is required.

Because after avoir envie de, French uses the infinitive.

So:

• nous avons envie de marcher = we feel like walking

Here, marcher is not the main conjugated verb.
The conjugated verb is avons from avoir.

Structure:

• nous avons = we have
• envie de marcher = the desire to walk / like walking

If you said marchons, that would be a conjugated form meaning we walk or a command let’s walk, which does not fit here.

They are similar, but not exactly the same.

• nous avons envie de marcher = we feel like walking
• nous voulons marcher = we want to walk

Avoir envie de often sounds a bit softer, more natural for a feeling or mood.
Vouloir can sound stronger or more direct.

In this sentence, avons envie de marcher fits well because it expresses the pleasant urge or desire to walk after work.

Because après here is followed by a noun phrase:

• le travail = work

So:

• après le travail = after work / after work is finished

This is a very common expression.

You generally would not say après travailler in this sentence. That would not be the normal structure.

Compare:

• après le dîner = after dinner
• après les cours = after class
• après le travail = after work

French can sometimes use après + infinitive, but that is a different structure and often appears in more formal grammar as après avoir travaillé = after working / after having worked.

For everyday French, après le travail is the natural choice here.

French often uses an article where English does not.

So:

• après le travail = after work

Even though English says simply work, French normally says le travail.

This is very common with abstract nouns and routine activities:

• Je vais au travail. = I’m going to work.
• Elle est au travail. = She’s at work.
• Après le travail = after work

So the le is normal and necessary here.

Yes, absolutely. In everyday spoken French, on is often used instead of nous.

So:

• nous avons envie de marcher = more standard/written
• on a envie de marcher = very common in speech

Both mean we feel like walking.

This sentence uses nous, which is perfectly correct and slightly more formal or written in tone.

In plus longs meaning longer, the s in plus is usually pronounced because the next word begins with a consonant.

So it sounds roughly like:

• plu long

not like plooz long.

A useful rule of thumb:

• before a consonant: plus often sounds like plu
• before a vowel, there may be a liaison:
  • plus intéressant may sound like plu zintéressant

Pronunciation can vary a bit depending on speaker and style, but in plus longs, learners will usually hear the final s as silent.

The comma separates the time expression from the main clause.

• En mars, les jours deviennent plus longs...

This is similar to English:

• In March, the days become longer...

The comma is not always absolutely required in every short sentence, but it is very natural and helps readability. It marks En mars as an introductory time phrase.