En novembre, il y a de moins en moins de feuilles dans le jardin.

En novembre means in November or during November.

French commonly uses en with months:

• en janvier = in January
• en avril = in April
• en novembre = in November

So here it sets the time frame right away.

With months, standard French uses en.

So you say:

• en novembre
• en décembre
• en été (in summer)

You do not normally say au novembre.
Dans novembre is also not standard for simply saying in November.

Il y a means there is or there are.

In this sentence:

• il y a ... de feuilles = there are ... leaves

It is one of the most common French expressions for saying that something exists or is present somewhere.

Because il y a is a fixed expression.

It stays il y a whether the thing after it is singular or plural:

• Il y a une feuille = There is one leaf
• Il y a des feuilles = There are leaves

The verb does not change here to match feuilles. You always use il y a in the present tense.

De moins en moins de means less and less or, with countable nouns in natural English, often fewer and fewer.

So:

• de moins en moins de feuilles = fewer and fewer leaves

This expression shows a gradual decrease over time.

Compare:

• de plus en plus de feuilles = more and more leaves
• de moins en moins de feuilles = fewer and fewer leaves

This is the normal structure.

The full pattern is:

de moins en moins de + noun

Examples:

• de moins en moins de temps = less and less time
• de moins en moins d’eau = less and less water
• de moins en moins de feuilles = fewer and fewer leaves

The first de is part of the whole quantity expression, and the second de introduces the noun that is being counted or measured.

After expressions of quantity or comparison, French usually uses de, not des.

So you get:

• beaucoup de feuilles = a lot of leaves
• moins de feuilles = fewer leaves
• de moins en moins de feuilles = fewer and fewer leaves

You would not normally say de moins en moins des feuilles here.

Because feuilles is a countable noun: you can count leaves.

In careful English:

• less is usually for uncountable things: less water, less time
• fewer is for countable things: fewer leaves, fewer books

So the most natural translation is:

• fewer and fewer leaves

But French uses moins for both less and fewer, depending on context.

Dans le jardin means in the garden.

Breakdown:

• dans = in / inside
• le jardin = the garden

French uses the definite article le here just as English uses the in the garden.

Sometimes, but it is not the same as the most neutral choice here.

• dans le jardin clearly means in the garden
• au jardin can sometimes be used, but it is less literal and less common in a basic sentence like this

For a learner, dans le jardin is the safest and most standard choice here.

The comma separates the introductory time phrase from the rest of the sentence.

• En novembre, il y a...

This is similar to English:

• In November, there are...

The comma is natural because En novembre has been moved to the front for emphasis or clarity. In short sentences, punctuation can sometimes vary, but this comma is completely normal.

Yes. That would also be correct, but the meaning changes slightly.

• il y a moins de feuilles = there are fewer leaves
• il y a de moins en moins de feuilles = there are fewer and fewer leaves

The second version emphasizes a continuing decline, as if the number keeps dropping over time. That makes it especially suitable for autumn.

The singular is feuille, meaning leaf.

• une feuille = a leaf
• des feuilles = leaves

In this sentence it is plural because the idea is about leaves in general, not just one leaf.