Mi hija se agarra a la barandilla cuando el suelo está mojado.

Because agarrarse a is the usual pattern for to hold on to something.

• agarrar algo = to grab something
• agarrarse a algo = to hold on to something / cling to something

So in this sentence, se is part of the verb expression agarrarse a. It shows that the daughter is holding herself onto the railing for support, not just grabbing it briefly.

Because the verb pattern is agarrarse a + thing/person.

So:

• se agarra a la barandilla
• se agarra a mi brazo
• se agarró a la puerta

That a belongs to the verb structure, not to the object by itself. English uses to in hold on to, and Spanish uses a here.

Because barandilla is a feminine noun: la barandilla.

Also, Spanish only contracts a + el = al.
It does not contract a + la.

So:

• a + el suelo → al suelo
• a + la barandilla → a la barandilla

Barandilla means railing, banister, or handrail, depending on context.

It is the protective rail you hold onto on stairs, balconies, ramps, etc. In this sentence, railing or handrail is the most natural idea.

Spanish uses definite articles very naturally with nouns that refer to familiar, concrete things in a situation.

Here, la barandilla and el suelo sound completely normal because we are talking about:

• the railing available in that setting
• the floor as the surface underfoot

Spanish often uses the article where English also does, but sometimes even more readily than English. So agarrarse a la barandilla and cuando el suelo está mojado sound natural and standard.

Here suelo means floor.

In Spanish, suelo can mean the surface under your feet, so depending on context it can be:

• floor indoors
• ground more generally

In this sentence, because we are talking about a slippery surface and a railing, floor is the most likely meaning.

Because mojado describes a temporary state, so Spanish uses estar, not ser.

• estar mojado = to be wet
• ser mojado would sound wrong here

This is a classic ser vs estar distinction:

• ser = essential characteristics, identity
• estar = states, conditions, results

A floor is not inherently wet; it is wet at that moment or when that condition happens, so está mojado is correct.

Because mojado agrees with el suelo, which is masculine singular.

Agreement in Spanish works like this:

• el suelo está mojado
• la barandilla está mojada
• los suelos están mojados
• las barandillas están mojadas

So the adjective must match the noun it describes.

Because this sentence describes a habitual or general situation: whenever the floor is wet, my daughter holds onto the railing.

For repeated, usual, or generally true situations, Spanish uses the indicative:

• cuando el suelo está mojado

If you were talking about a future or not-yet-real situation, Spanish would often use the subjunctive:

• cuando el suelo esté mojado, ten cuidado
  = when the floor is wet / when it becomes wet, be careful

So here está is used because the sentence describes a regular pattern.

Yes. The present tense in Spanish is often used to talk about habits or things that usually happen.

So Mi hija se agarra a la barandilla cuando el suelo está mojado means something like:

• My daughter holds onto the railing when the floor is wet
• Whenever the floor is wet, my daughter holds onto the railing

It is not necessarily happening right now; it can describe a usual behavior.

Yes, but it is slightly different.

• Mi hija agarra la barandilla = my daughter grabs the railing
• Mi hija se agarra a la barandilla = my daughter holds onto the railing / grabs onto it for support

The version with se and a sounds more natural when the idea is balance, safety, or support.

Yes, it is understood and used in Spain. In this context it sounds natural.

In Spain, you may also hear:

• cogerse a la barandilla
• sujetarse a la barandilla

They are close in meaning, though the nuance can differ slightly:

• agarrarse = grab/hold on
• cogerse = take hold of / hold on
• sujetarse = hold oneself steady / support oneself

For a learner, agarrarse a la barandilla is a very good, natural phrase.

Yes, if the context already makes it clear who you are talking about.

Spanish often leaves out the subject when it is understood:

• Se agarra a la barandilla cuando el suelo está mojado.

But in your sentence, Mi hija is included because the speaker wants to identify who does the action. That is completely normal.

The sentence uses the most neutral and easy-to-understand word order:

Mi hija + se agarra a la barandilla + cuando el suelo está mojado

Spanish word order is fairly flexible, but this is the standard order for a simple statement. You could move parts around for emphasis, for example:

• Cuando el suelo está mojado, mi hija se agarra a la barandilla.

This version emphasizes the condition first, but the meaning stays the same.