Si el cordón de tus zapatillas se suelta, tienes que atarlo otra vez.

Because after si for a real, possible condition in the present or future, Spanish normally uses the present indicative, not the subjunctive.

So:

• Si el cordón ... se suelta = if the lace comes undone / if the lace gets loose

This is a normal real-life condition, so se suelta is the expected form.

You would not say si ... se suelte here.

Soltarse is a very common pronominal verb. Here it means something like:

• to come loose
• to come undone

So el cordón se suelta means the lace loosens/comes undone by itself, rather than someone actively loosening it.

Compare:

• Suelto el cordón = I loosen the lace
• El cordón se suelta = the lace comes loose

That se often gives the idea that the thing undergoes the change itself.

Because the sentence is talking about one lace. Even though you have two trainers, the speaker is imagining one specific lace becoming loose.

So:

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

If you wanted to talk about laces in general, you could also say:

• los cordones de tus zapatillas

Both are possible, depending on whether you mean one lace or the laces in general.

Yes. That would also sound natural.

The difference is mainly one of focus:

• el cordón = one lace, a single lace
• los cordones = the laces, more generally

So these are both fine:

• Si el cordón de tus zapatillas se suelta...
• Si los cordones de tus zapatillas se sueltan...

The rest of the sentence would then also need to match in number:

• ...tienes que atarlo otra vez
• ...tienes que atarlos otra vez

Yes, debes is possible, but tienes que is very common in everyday Spanish for obligation.

• tienes que atarlo = you have to tie it
• debes atarlo = you should / must tie it

In many contexts, tienes que sounds a bit more neutral and conversational.
Debes can sound slightly more formal or stronger, depending on context.

Lo is the direct object pronoun meaning it, and it refers to el cordón.

So:

• atar = to tie
• atarlo = to tie it

Because cordón is:

• masculine
• singular

the pronoun is lo.

If the noun were feminine, you would use la instead.

Yes. Both are correct:

• tienes que atarlo
• lo tienes que atar

Spanish allows object pronouns to go:

1. attached to the infinitive
2. before the conjugated verb

So both versions are grammatical and natural.
In everyday speech, both are very common.

Because atar takes a direct object here, not an indirect object.

You tie the lace, so the lace is the thing directly affected by the action:

• atar + direct object

That is why Spanish uses:

• lo for masculine singular
• not le

So:

• atarlo = tie it

Using atarle would not fit this structure.

Both can work, but they are slightly different in feel.

• soltarse = to come loose
• desatarse = to come untied / to become untied

With shoelaces, desatarse is very common because a lace typically becomes untied.
Soltarse is also natural and can suggest that it has loosened or come undone.

So you may also hear:

• Si el cordón de tus zapatillas se desata...

That is very natural too.

Yes, both are possible.

• otra vez = again
• de nuevo = again / anew

In this sentence, otra vez is a very common everyday choice.
De nuevo is also correct and sounds slightly more formal or stylistically different in some contexts.

So both work:

• tienes que atarlo otra vez
• tienes que atarlo de nuevo

Yes, zapatillas is very natural in Spain for trainers/sneakers, especially in everyday language.

Depending on the country, other words may be more common, such as:

• tenis
• zapatos deportivos
• zapatillas deportivas

So if you are learning Spanish from Spain, zapatillas is a very useful word to know.

Yes, and that is actually very idiomatic.

• Si se te suelta el cordón de las zapatillas...

The te here is often called an ethical dative or dative of involvement. It shows that this happens to you or affects you, even though el cordón is still the subject.

It does not mean you loosen it. It means something like:

• the lace comes loose on you

This version is very common in natural spoken Spanish.

Yes, you could say tu cordón, but el cordón de tus zapatillas is more specific and clearer.

• el cordón de tus zapatillas = the lace of your trainers
• tu cordón = your lace

The version with el + de tus zapatillas identifies exactly what cord is meant.
That is especially useful because cordón can mean different kinds of cord or lace in other contexts.

So the article el is just the normal definite article here: the lace.

It is standard and helpful.

The first part:

• Si el cordón de tus zapatillas se suelta

is a conditional clause. In Spanish, when that clause comes first, it is normally followed by a comma before the main clause:

• ..., tienes que atarlo otra vez.

This is the usual punctuation and makes the sentence easier to read.

In many Spanish-speaking countries, yes, but in Spain atar is the more standard everyday choice here.

• atar = very common in Spain for tying
• amarrar = more common in some other regions, and can sound regional or less typical in Spain for shoelaces

So for Spanish from Spain, atarlo is the best choice.