Es mejor quedarse en casa hasta que deje de llover.

Es mejor uses ser because it is making a general judgment: it is better.

• ser is used for general statements, definitions, and evaluations.
• estar mejor usually means to be feeling better or to be in a better condition.

So here:

• Es mejor quedarse en casa = It is better to stay at home
• Está mejor = He/She is better or It is in better condition

Because quedarse means to stay / to remain, which is the meaning needed here.

Compare:

• quedar can mean to arrange to meet, to be left, to fit, or to remain in some contexts
• quedarse very often means to stay behind / remain somewhere

So:

• quedarse en casa = to stay at home

This is a very common reflexive verb in Spanish. Even though English just says stay, Spanish usually uses the reflexive form here.

Because quedarse is in the infinitive, and the sentence is using an impersonal structure.

Es mejor + infinitive means It is better to...

So:

• Es mejor quedarse en casa = It is better to stay at home

This structure does not need an explicit subject. It can mean:

• it is better for people in general
• it is better for us
• it is better for one

If you want to be more specific, you could say:

• Es mejor que nos quedemos en casa = It is better that we stay at home

That version uses a finite verb instead of the infinitive.

In Spanish, en casa is the normal way to say at home or at the house/home in a general sense.

• en casa = at home
• en la casa = in the house

So in this sentence, en casa sounds natural because it refers to home as the place where one stays.

Compare:

• Estoy en casa = I’m at home
• Estoy en la casa de mi amigo = I’m in my friend’s house

Hasta que means until.

In this sentence:

• hasta que deje de llover = until it stops raining

It introduces the point in time when the action changes. So the full sentence means that staying at home is better up to the moment when the rain stops.

Because after hasta que, Spanish often uses the subjunctive when the action refers to the future or something that has not happened yet.

Here, the rain has not stopped yet, so Spanish uses the present subjunctive:

• deje = present subjunctive of dejar

So:

• hasta que deje de llover = until it stops raining

If the action were habitual or already known as a fact in the past, Spanish might use the indicative in other contexts. But in a sentence like this, referring to a future event, the subjunctive is the normal choice.

Because Spanish uses the expression dejar de + infinitive, which means to stop doing something.

So:

• dejar de llover = to stop raining
• literally: to cease from raining

Other examples:

• dejar de fumar = to stop smoking
• dejar de hablar = to stop talking

So deje de llover means it stops raining.

Because it depends on dejar de.

As mentioned above, dejar de + infinitive is a fixed structure:

• dejar de + verb

So:

• deje de llover
• stop + raining

In Spanish, when one verb follows another in this kind of structure, the second verb often stays in the infinitive.

The subject is understood, and with llover it is impersonal.

Llover means to rain, and weather verbs usually do not have a real personal subject in Spanish.

So:

• llueve = it rains / it is raining
• deje de llover = it stops raining

The it in English is only a grammatical placeholder. Spanish normally does not need to say it.

Yes, but it is not exactly the same.

• quedarse en casa = to stay at home
• estar en casa = to be at home

Quedarse emphasizes remaining there instead of going out. That fits the idea of waiting until the rain stops.

So in this sentence, quedarse en casa is more natural because it suggests:

• don’t go out
• stay where you are until the rain stops

Yes. Hasta que pare de llover is also correct and natural.

Two common ways to say until it stops raining are:

• hasta que deje de llover
• hasta que pare de llover

Both use the subjunctive after hasta que.

A small difference in feel:

• dejar de llover is very common and neutral
• parar de llover is also common and straightforward

In many contexts, they are interchangeable.

Not necessarily. By itself, it is general and impersonal:

• Es mejor quedarse en casa... = It is better to stay at home...

Depending on context, it could mean:

• we should stay at home
• you should stay at home
• people should stay at home

If you want to make we explicit, you could say:

• Es mejor que nos quedemos en casa hasta que deje de llover

That clearly means It’s better that we stay at home until it stops raining.