No puedo dejar de mirar el cielo cuando está lleno de estrellas.

In Spanish, the subject pronoun is often left out because the verb ending already tells you who the subject is.

• puedo = I can
• So No puedo already clearly means I can’t

You could say Yo no puedo, but adding yo usually gives extra emphasis, like:

• Yo no puedo dejar de mirar... = I can’t stop looking...

Without that emphasis, No puedo sounds more natural.

Dejar de + infinitive is a very common structure meaning to stop doing something.

So:

• dejar de mirar = to stop looking
• No puedo dejar de mirar = I can’t stop looking

This is an important pattern to learn:

• dejar de fumar = to stop smoking
• dejar de hablar = to stop talking
• dejar de pensar en algo = to stop thinking about something

Here, the idea is not literally leave looking, but stop looking.

Because dejar de + infinitive is a fixed expression.

• dejar on its own can mean to leave, to let, or to stop
• But dejar de + infinitive specifically means to stop doing

Compare:

• Dejo el libro en la mesa = I leave the book on the table
• Me deja entrar = He/she lets me come in
• Dejo de leer = I stop reading

So in your sentence, the de is required.

No. No puedo dejar mirar el cielo does not mean the same thing.

Without de, dejar + infinitive usually means to allow/let someone do something, and it normally needs another person as the object:

• No me deja mirar el cielo = He/she doesn’t let me look at the sky

But your sentence is:

• No puedo dejar de mirar el cielo = I can’t stop looking at the sky

So de completely changes the meaning.

Both can relate to seeing, but they are not exactly the same.

• ver = to see
• mirar = to look at

In this sentence, mirar fits better because it suggests an active, deliberate action: you keep looking at the sky.

Compare:

• Veo el cielo = I see the sky
• Miro el cielo = I look at the sky

Since the sentence is about being unable to stop doing it, mirar sounds more natural than ver.

Spanish often uses the definite article more than English does.

So where English says:

• the sky

Spanish naturally says:

• el cielo

In this sentence, mirar el cielo is the normal way to say to look at the sky. Leaving out el would sound incomplete or unnatural here.

Because lleno describes a condition or state here, not an essential characteristic.

Spanish often uses:

• estar for states, conditions, and results
• ser for identity, classification, or more permanent characteristics

So:

• está lleno de estrellas = it is full of stars

This means the sky is in that state at that moment or in that situation.

Compare:

• La sala está llena = The room is full
• Es grande = It is big

Using es lleno here would sound wrong.

Lleno de means full of.

So:

• lleno = full
• de estrellas = of stars

Together:

• lleno de estrellas = full of stars

This structure is very common:

• lleno de gente = full of people
• lleno de agua = full of water
• lleno de ideas = full of ideas

Because after lleno de, Spanish often uses a noun without an article when speaking in a general sense.

So:

• lleno de estrellas = full of stars

This sounds natural and general. You are not talking about a specific set of stars, just stars in general.

You could sometimes use an article in other contexts for a more specific meaning, but here de estrellas is the normal choice.

It can be understood as either, depending on context.

• when if you mean a particular occasion
• whenever if you mean this happens every time the sky is full of stars

In this sentence, many learners will naturally understand it as something close to whenever:

• I can’t stop looking at the sky whenever it is full of stars

That is because the sentence expresses a general habit or repeated experience.

The hidden subject of está is el cielo.

The sentence breaks down like this:

• No puedo dejar de mirar el cielo = I can’t stop looking at the sky
• cuando está lleno de estrellas = when it is full of stars

Spanish often leaves out subject pronouns like it, because the meaning is understood from context. Here, está refers back to el cielo.

So in English you need it, but in Spanish you do not.

Yes, but the original order is very natural.

For example, you could also say:

• Cuando el cielo está lleno de estrellas, no puedo dejar de mirarlo.

This means the same thing, though now the sentence begins with the time clause. Notice that in this version, mirarlo uses lo to refer back to el cielo.

The original version is simpler and very natural:

• No puedo dejar de mirar el cielo cuando está lleno de estrellas.

Because the speaker uses the full noun phrase el cielo instead.

Both patterns are possible depending on how you build the sentence:

• mirar el cielo = look at the sky
• mirarlo = look at it

In the original sentence, the noun is stated directly:

• No puedo dejar de mirar el cielo...

If the sky had already been mentioned, a speaker might say:

• No puedo dejar de mirarlo cuando está lleno de estrellas.

Here, lo stands for el cielo.

Dejar de + infinitive is extremely common and very natural in Spanish.

You may also hear:

• parar de + infinitive
• no poder parar de + infinitive

For example:

• No puedo parar de mirar el cielo

That also means I can’t stop looking at the sky. It is natural too, but dejar de is a very standard and widely used structure that learners should know well.