Si el cordón de tus zapatillas está flojo, tienes que atarlo otra vez.

Si means if. It introduces a condition:

Si el cordón de tus zapatillas está flojo... = If the shoelace of your trainers is loose...

This is a very common Spanish pattern:

• Si
  • present tense
• main clause in the present tense, often expressing a consequence, instruction, or obligation

So Si... tienes que... means If..., you have to...

El cordón is singular, so it means the lace or the shoelace. Spanish is talking about one lace.

If you wanted to talk about both laces, you would normally say:

Si los cordones de tus zapatillas están flojos, tienes que atarlos otra vez.

So the original sentence is perfectly correct; it is just focusing on one shoelace.

Here cordón means shoelace.

On its own, cordón can also mean cord, lace, or string, depending on context. In this sentence, because it is connected to zapatillas, the meaning is clearly shoelace.

In Spain, zapatillas often means trainers / sneakers.

So:

• zapatillas = trainers, sneakers

Be aware that in some other Spanish-speaking countries, zapatillas can mean something different, such as slippers or other kinds of shoes. But in Spain, zapatillas for sports shoes is very common.

De tus zapatillas means of your trainers or more naturally in English, your trainers’.

Spanish often uses this structure:

• noun + de + noun

So:

• el cordón de tus zapatillas = the shoelace of your trainers

Also notice that tus agrees with zapatillas, which is plural:

• tu zapatilla = your trainer
• tus zapatillas = your trainers

It uses está because being loose is a temporary state or condition.

• estar flojo = to be loose, slack
• ser flojo would usually suggest a more permanent quality, and with people it can even mean lazy or weak

So for a shoelace that is loose right now, está flojo is the natural choice.

Because flojo agrees with el cordón, which is masculine singular.

Agreement in Spanish:

• el cordón → masculine singular
• adjective must match → flojo

If the noun were feminine, the adjective would change:

• la cuerda está floja

If it were plural:

• los cordones están flojos

Tienes que means you have to or you need to.

It comes from:

• tener que + infinitive = to have to do something

So:

• tienes que atarlo = you have to tie it

This is one of the most common ways to express obligation in Spanish.

Because when a pronoun goes with an infinitive, Spanish often attaches it to the end.

• atar = to tie
• lo = it
• atarlo = to tie it

Here lo refers to el cordón.

You can also say:

• lo tienes que atar

Both are correct:

• tienes que atarlo
• lo tienes que atar

The meaning is the same.

It is lo because it refers to el cordón, which is masculine singular, and it is the direct object of atar.

• atar el cordón = to tie the shoelace
• replace el cordón with a pronoun → atarlo

Why not la?

• because cordón is masculine, not feminine

Why not le?

• because le is normally used for an indirect object, not the direct object here

Otra vez means again or once again.

So:

• atarlo otra vez = to tie it again

A very similar expression is de nuevo, which also means again:

• tienes que atarlo de nuevo

Both are natural.

In many Spanish-speaking countries, yes, amarrar can mean to tie. But in Spain, atar is the more usual and neutral choice in this context.

So for Spain Spanish:

• atar el cordón sounds the most natural

Yes, it is correct and understandable. It is a clear, standard sentence.

In everyday speech, though, Spanish speakers might also say something slightly more idiomatic, such as:

• Si se te ha desatado el cordón, tienes que volver a atarlo.
• Si tienes el cordón suelto, tienes que atarlo otra vez.

But the original sentence is absolutely fine for learners and good Spanish.