Derrière la maison, il y a une terrasse en bois et une petite allée qui mène au portail.

Il y a means there is or there are. It is one of the most common French expressions for saying that something exists or is present somewhere.

In this sentence, il y a une terrasse... et une petite allée... means there is a wooden terrace and a small path...

Literally, il y a comes from something like it has there, but you should learn it as a fixed expression meaning there is/there are.

Both terrasse and allée are feminine nouns, so they take the feminine article une.

That is also why the adjective is petite and not petit. French adjectives usually agree in gender and number with the noun they describe.

So:

• un petit jardin = a small garden
• une petite allée = a small path

Allée usually means a path, walkway, drive, or lane, depending on context. In this sentence, it suggests a small path leading to the gate.

It does not always mean a road for cars. Here, une petite allée sounds more like a small walkway or garden path.

In French, en is often used to indicate the material something is made of, especially in everyday descriptions.

So:

• une table en bois = a wooden table
• une statue en pierre = a stone statue
• une bague en or = a gold ring

So une terrasse en bois means a wooden terrace or a terrace made of wood.

French uses qui here to introduce the relative clause: qui mène au portail.

English can say a small path that leads to the gate. In French, qui does that job.

So:

• une petite allée qui mène au portail = a small path that leads to the gate

French often requires the relative pronoun where English might sometimes omit it.

Use qui when the relative pronoun is the subject of the verb that follows.

Here, the thing doing the action of mène is une petite allée. The path leads to the gate. So qui is correct.

A quick comparison:

• Le livre qui est sur la table = The book that is on the table
  (qui is the subject of est)
• Le livre que je lis = The book that I am reading
  (que is the object of lis)

Mène comes from the verb mener, which means to lead.

So qui mène au portail means that leads to the gate.

The spelling mène with è is part of the verb’s normal present-tense stem change:

• je mène
• tu mènes
• il/elle mène
• ils/elles mènent

The accent shows the vowel sound changes in these forms.

Because à + le contracts to au in French.

So:

• à le portail becomes au portail
• à les becomes aux

That is a standard contraction:

• Je vais au parc = I am going to the park
• Il mène au portail = It leads to the gate

Portail usually means a gate, especially a larger or more formal one, such as a garden gate, entrance gate, or driveway gate.

It is not the same as the English word portal in most normal situations. Even though they look similar, in everyday French portail usually refers to a physical gate.

Une is the indefinite article, meaning a. It is used when introducing something that has not been specifically identified before.

So:

• une terrasse = a terrace
• une petite allée = a small path

If the speaker were referring to specific, already known things, French might use la instead:

• la terrasse
• la petite allée

Here, the sentence is simply describing what is there, so the indefinite article is natural.

The comma marks the opening location phrase and separates it from the main clause. It helps readability and reflects a slight pause.

So the sentence is structured like this:

• Derrière la maison, = location setting
• il y a... = main statement

In short sentences, punctuation can vary a little, but the comma is very normal here.

Yes, often it could. Terrasse can mean terrace or patio, depending on context. In a house description like this, une terrasse en bois could easily be understood as a wooden patio or a wooden deck, depending on what it looks like.

So the exact English word may change, but the French is perfectly natural.