Al llegar a casa, me desato los cordones y dejo las zapatillas junto a la puerta.

Al + infinitive is a very common Spanish structure meaning when, upon, or on doing something.

So:

• al llegar a casa = when I get home / upon arriving home

It is literally:

• a + el = al
• llegar = to arrive

This structure is often used to introduce an action that happens immediately before another one.

Examples:

• Al entrar, saludó a todos. = When he came in, he greeted everyone.
• Al terminar, apaga la luz. = When you finish, turn off the light.

In your sentence, it sets the scene for what the speaker usually does after getting home.

In Spanish, casa often appears without an article when it means home in a general, personal sense.

So:

• llegar a casa = to get home
• estar en casa = to be at home
• volver a casa = to return home

If you say la casa, you are usually talking about the house/building itself, more literally.

Compare:

• Llego a casa a las seis. = I get home at six.
• Llego a la casa de mi abuela a las seis. = I arrive at my grandmother’s house at six.

So in your sentence, a casa is the natural way to say home.

The me shows that the action is being done to oneself.

• desatar = to untie
• desatarse = to untie something on oneself / to untie one’s own...

So:

• me desato los cordones = literally, I untie myself the shoelaces
• natural English: I untie my shoelaces

This is very common in Spanish with actions involving body parts, clothing, and personal items being worn or used by the subject.

Similar examples:

• Me lavo las manos. = I wash my hands.
• Me quito la chaqueta. = I take off my jacket.
• Me cepillo los dientes. = I brush my teeth.

English usually uses my, but Spanish often prefers a reflexive pronoun + definite article.

Because in Spanish, when it is already clear whose thing it is, the language often uses:

• a reflexive pronoun (me, te, se...)
• plus the definite article (el, la, los, las)

So:

• me desato los cordones literally = I untie the shoelaces on myself
• natural English = I untie my shoelaces

Spanish does not need mis here, because me already tells us they are the speaker’s shoelaces.

Compare:

• Me pongo la camisa. = I put on my shirt.
• Se lava la cara. = He/She washes his/her face.

You could use a possessive in some contexts for emphasis or contrast, but here los cordones is the normal choice.

Here desatar means to untie.

So:

• desatar los cordones = to untie the shoelaces

The basic verb atar means to tie. Adding des- often gives the opposite meaning:

• atar = tie
• desatar = untie

With shoelaces, Spanish speakers commonly say:

• atarse los cordones = to tie one’s shoelaces
• desatarse los cordones = to untie one’s shoelaces

It is the normal everyday verb in this context.

In Spain, zapatillas can mean different kinds of casual footwear depending on context.

Common possibilities include:

• trainers / sneakers
• slippers

In this sentence, because the person is undoing the laces and leaving them by the door, zapatillas most likely means trainers / sneakers.

This is a good example of how footwear words vary a lot across Spanish-speaking countries. In Spain:

• zapatillas deportivas = sports shoes / trainers
• zapatillas de casa = slippers

If no extra description is given, context usually tells you which meaning is intended.

Because the sentence is focusing on where the shoes end up, not just on the act of taking them off.

• me quito las zapatillas = I take off my shoes
• dejo las zapatillas junto a la puerta = I leave the shoes next to the door

In your sentence, both ideas are present implicitly:

1. the speaker unties the laces
2. the speaker leaves the shoes by the door

Spanish does not need to explicitly say I take them off if that is already obvious from the situation.

Junto a la puerta means next to the door or by the door.

It is very similar to:

• al lado de la puerta = next to the door

In most everyday contexts, they mean almost the same thing.

A rough difference:

• junto a can sound a little more compact or slightly more formal/written
• al lado de is extremely common in everyday speech

So both are good:

• Dejo las zapatillas junto a la puerta.
• Dejo las zapatillas al lado de la puerta.

The meaning is basically the same here.

The present tense in Spanish is often used for habitual actions or routines.

So this sentence sounds like:

• When I get home, I untie my laces and leave my shoes by the door.

It describes something the speaker normally does, not necessarily only what they are doing right now.

This is the same as in English when you say:

• When I get home, I make tea.
• On Sundays, I sleep late.

If the speaker wanted to describe one specific past occasion, they would use past tenses instead.

Yes, absolutely.

These are both natural:

• Al llegar a casa, me desato los cordones...
• Cuando llego a casa, me desato los cordones...

The difference is mostly one of style and structure:

• Al llegar a casa is more compact and slightly more elegant/written in tone
• Cuando llego a casa is more explicit and very common in speech

Both mean when I get home in this sentence.

The opening phrase Al llegar a casa works as an introductory time expression, so it is natural to separate it with a comma:

• Al llegar a casa, me desato los cordones...

The comma helps show:

1. the time-setting part
2. the main action

In short sentences, Spanish punctuation can sometimes be flexible, but with an introductory phrase like this, the comma is very standard and helpful.

The word order here is the most neutral and natural one.

Breakdown:

• me desato los cordones = I untie my shoelaces
• y dejo las zapatillas junto a la puerta = and leave my shoes by the door

Spanish word order is often flexible, but this version sounds straightforward and idiomatic.

For example, you could move some parts for emphasis, but it would sound less neutral:

• Junto a la puerta dejo las zapatillas.
• Las zapatillas las dejo junto a la puerta.

Those are possible, but the original sentence is the normal everyday order.