Una sandalia tiene la suela un poco rota, así que es mejor no usarla hoy.

Because Spanish often uses the definite article for parts of something when the owner is already clear from context.

So in Una sandalia tiene la suela un poco rota, it is already obvious that the sole belongs to the sandal. Spanish prefers la suela rather than su suela.

This is similar to other common patterns:

• Me duele la cabeza = My head hurts
• El coche tiene la puerta abierta = The car has the door open

Using su suela would sound less natural here unless you really needed to emphasize whose sole you mean.

Because rota agrees with la suela, which is a feminine singular noun.

• la suela → feminine singular
• adjective must match → rota

Compare:

• el zapato está roto
• la sandalia está rota
• la suela está rota

Even though sandalia is also feminine, the adjective rota here is directly describing suela, not sandalia.

Spanish can use both, but they mean slightly different things structurally.

• Una sandalia tiene la suela un poco rota
  = A sandal has the sole a little broken

This pattern focuses on one part of the object.

• La suela de una sandalia está un poco rota
  = The sole of a sandal is a little broken

This says more directly that the sole is broken.

• Una sandalia está un poco rota
  = A sandal is a little broken

This refers to the sandal as a whole, not specifically the sole.

So tiene la suela rota is a very natural way to say that one part of something is damaged.

Because un poco modifies the adjective rota.

• un poco rota = a little broken

That is the normal order in Spanish: degree expression + adjective.

Compare:

• un poco cansado = a little tired
• muy difícil = very difficult
• bastante grande = quite big

Saying rota un poco would not work here for the same meaning.

Así que means so, therefore, or so then. It introduces a result or conclusion.

In this sentence:

• Una sandalia tiene la suela un poco rota, así que es mejor no usarla hoy.
• The sole is damaged, so it is better not to wear/use it today.

It is very common in everyday Spanish.

Other similar connectors are:

• por eso = for that reason / that’s why
• entonces = then / so
• de modo que = so / therefore

Among these, así que is very natural and conversational.

Because after es mejor, Spanish normally uses an infinitive to express what is better to do.

• Es mejor no usarla = It’s better not to use/wear it

And object pronouns with an infinitive usually go attached to the end:

• usar + la = usarla

You could also put the pronoun before a conjugated verb, but here there is no conjugated verb after es mejor. The verb is an infinitive, so attachment is the normal choice.

Compare:

• Quiero comprarlo
• Prefiero no llevarla
• Es mejor arreglarla

It refers to una sandalia.

• sandalia is feminine singular
• so the direct object pronoun is la

So:

• usar la sandalia → usarla

In context, Spanish uses the pronoun because repeating la sandalia would sound less natural:

• es mejor no usar la sandalia hoy is possible
• es mejor no usarla hoy is more natural

Grammatically, la could be feminine singular, so in isolation it might seem ambiguous. But in this sentence, common sense makes it clear that usarla refers to the sandal, not the sole.

You use/wear the sandal, not the sole by itself.

So the listener naturally understands:

• broken sole → better not wear the sandal

Usar is correct and fully understandable, but in Spain, people often prefer llevar or sometimes ponerse, depending on the exact meaning.

• usar = to use / to wear
• llevar = to wear
• ponerse = to put on

So in Spain, these may sound more idiomatic:

• ... así que es mejor no llevarla hoy.
• ... así que es mejor no ponérsela hoy.

The last one, ponérsela, means not to put it on today.

So your sentence is fine, but for Spain specifically, llevarla may sound a bit more natural than usarla when talking about footwear.

Spanish can use either singular or plural, depending on what the speaker wants to focus on.

• Una sandalia focuses on one sandal
• unas sandalias would usually mean a pair of sandals or sandals in general

So this sentence suggests that one of the sandals has a damaged sole:

• Una sandalia tiene la suela un poco rota

If you wanted to talk about the pair more generally, you might say:

• Las sandalias tienen la suela un poco rota
• Una de las sandalias tiene la suela un poco rota

That last option is especially clear if you mean only one of the two sandals.

Yes. Spanish allows several natural alternatives, depending on what you want to emphasize.

Some good options are:

• Una sandalia tiene la suela un poco rota, así que es mejor no usarla hoy.
• La suela de una sandalia está un poco rota, así que es mejor no usarla hoy.
• Una de las sandalias tiene la suela un poco rota, así que es mejor no llevarla hoy.

The original version is natural because it starts with the object, then mentions the damaged part, then gives the consequence. It is a very typical Spanish way to structure this idea.