Cuando estoy fuera de casa, siempre llevo el móvil y las llaves.

Because cuando without an accent is a conjunction meaning when in a statement:

Cuando estoy fuera de casa, siempre llevo el móvil y las llaves.

With an accent, cuándo is used in questions or indirect questions:

• ¿Cuándo vuelves? = When are you coming back?
• No sé cuándo vuelve. = I don’t know when he/she is coming back.

So here, because it introduces a normal time clause, cuando has no accent.

Because Spanish uses estar for temporary states and location, and fuera de casa describes where you are or your temporary situation.

• estoy fuera de casa = I am away from home / out of the house

Using ser here would sound wrong, because ser is for identity, origin, permanent characteristics, and similar ideas.

So:

• Estoy en casa = I’m at home
• Estoy fuera de casa = I’m away from home

It literally means outside of home/house, but in natural English it usually means away from home.

There is a small nuance:

• fuera de casa often means you are not at home
• fuera de la casa would sound more like physically outside the house/building

So in this sentence, fuera de casa is the normal idiomatic way to say away from home.

Because casa often appears without an article in common expressions when it means home rather than a specific building.

Very common patterns are:

• en casa = at home
• a casa = home
• de casa = from home / of home
• fuera de casa = away from home

If you say de la casa, you are usually talking about a specific house as a physical object:

• Estoy fuera de la casa = I am outside the house

So fuera de casa is the idiomatic choice here.

Because the sentence describes a habit or a general rule.

Spanish often uses the present tense for things that happen regularly:

• Cuando estoy fuera de casa, siempre llevo el móvil y las llaves.

This means something like:

• Whenever I’m away from home, I always carry my phone and keys.

It is not talking about one single moment right now. It is talking about what the speaker usually does.

Here llevar means to carry or to take with you.

So:

• llevo el móvil y las llaves = I carry / take my phone and keys with me

This is a very common verb for objects you have on you.

It is different from some related verbs:

• llevar = to carry / take along
• traer = to bring
• coger = to take / pick up
• usar = to use / wear

In this sentence, llevar is the natural choice because the idea is that you have those things with you when you are away from home.

Because in Spanish, it is very common to use the definite article instead of a possessive when it is already obvious whose thing it is.

Here, since the subject is I, Spanish naturally understands that the phone and keys are the speaker’s:

• llevo el móvil y las llaves

This sounds very natural.

You can say:

• llevo mi móvil y mis llaves

but that usually feels more explicit or emphatic, as if you want to stress that they are my phone and my keys.

So both are possible, but el móvil y las llaves is very natural Spanish.

Because people often carry a set of keys, not just one key.

So las llaves usually means my keys in the everyday sense.

If you meant only one key, you would say:

• la llave = the key

But in normal life, Spanish often uses the plural because house keys, car keys, and other keys are commonly carried together.

That is the most natural placement here.

• Siempre llevo el móvil y las llaves.

In Spanish, adverbs like siempre, nunca, ya, and también often go before the conjugated verb.

You may also hear other orders, but they can sound more marked or less neutral:

• Llevo siempre el móvil y las llaves.

This is possible, but siempre llevo is the more standard, natural order in this sentence.

Yes, móvil is the normal word in Spain for mobile phone / cell phone.

In many Latin American countries, people more often say:

• celular
• sometimes móvil, depending on the region

Since you are learning Spanish from Spain, el móvil is exactly the word you would expect here.

It is normal and appropriate here.

When a time clause like Cuando estoy fuera de casa comes at the beginning of the sentence, Spanish commonly separates it from the main clause with a comma:

• Cuando estoy fuera de casa, siempre llevo el móvil y las llaves.

If you put that clause at the end, the comma is usually not needed:

• Siempre llevo el móvil y las llaves cuando estoy fuera de casa.

So the comma in the original sentence is standard.

Yes, you could. It is grammatical and natural.

But there is a slight difference:

• fuera = out / outside / away
• fuera de casa = away from home

Without de casa, the sentence is a bit more general. It could mean simply when I’m out. With de casa, the meaning is clearer: when I’m away from home.

So the original version is more precise.