Dejé los globos junto a la silla para que no subieran al techo.

Dejé is the preterite of dejar, so it presents the action as a completed event: I left / I put the balloons there.

In this sentence, the speaker is talking about one specific action in the past:

• Dejé los globos junto a la silla... = I left the balloons next to the chair...

If you used dejaba, that would usually suggest a habit, background description, or an action seen as ongoing/repeated in the past:

• Dejaba los globos junto a la silla = I used to leave the balloons next to the chair / I was leaving the balloons next to the chair

So dejé fits best for a single completed action.

Spanish often uses the definite article more than English does. Los globos means the balloons, and it sounds natural when referring to specific balloons already known in the context.

In English, we might sometimes say just I left balloons..., but in Spanish that often sounds less natural unless you really mean some balloons in a more indefinite way.

Compare:

• Dejé los globos... = I left the balloons...
• Dejé unos globos... = I left some balloons...

So los tells you these are specific balloons, not just any balloons.

Junto a means next to, beside, or by.

So:

• junto a la silla = next to the chair

A very common alternative is:

• al lado de la silla

Both are natural. In many situations they mean basically the same thing.

Examples:

• Puse el libro junto a la lámpara.
• Puse el libro al lado de la lámpara.

Both mean I put the book next to the lamp.

Use para que when it is followed by a conjugated verb, especially when expressing purpose with a different subject.

Here:

• para que no subieran al techo
• literally: so that they wouldn’t rise to the ceiling

You cannot say para no subieran because after para, if a verb follows directly, it must normally be an infinitive:

• para no subir = in order not to go up

But in your sentence, the idea is:

• I left the balloons there so that the balloons would not rise...

That requires para que + subjunctive.

A useful rule:

• para + infinitive when the subject stays the same
• para que + subjunctive when the subject changes

Subieran is the imperfect subjunctive of subir.

It is used because:

1. para que triggers the subjunctive
2. the main verb dejé is in the past, so Spanish normally uses a past subjunctive after it

So:

• Dejé los globos... para que no subieran... = I left the balloons... so that they wouldn’t go up...

Why not the others?

• subieron = they went up → indicative, simple past; wrong after para que
• subirían = they would go up → conditional; also wrong here after para que

The structure needs subjunctive, not indicative or conditional.

The subjunctive is used because the clause after para que expresses a purpose, not a simple statement of fact.

The speaker is saying what they did in order to prevent something:

• I left the balloons next to the chair so that they wouldn’t rise to the ceiling.

That wouldn’t rise part is the intended result or goal, not a plain factual narration. Spanish typically uses the subjunctive in this kind of purpose clause.

So:

• para que no subieran = so that they would not rise

This is a very common pattern:

• Lo hice para que estuvieras bien. = I did it so that you would be okay.
• Cerró la puerta para que no entrara el frío. = He/She closed the door so that the cold wouldn’t come in.

Because the main verb is in a past tense: dejé.

A common sequence is:

• present/main present or future → present subjunctive
• past/main past → imperfect subjunctive

Compare:

• Dejo los globos junto a la silla para que no suban al techo.
  I leave the balloons next to the chair so that they don’t rise to the ceiling.

• Dejé los globos junto a la silla para que no subieran al techo.
  I left the balloons next to the chair so that they wouldn’t rise to the ceiling.

So subieran matches the past-time perspective created by dejé.

Because al is the contraction of a + el.

Since techo is masculine singular, the ceiling is:

• el techo

So:

• a + el techo → al techo

This contraction is required with a + el in normal Spanish:

• Voy al parque = I’m going to the park
• Subieron al techo = They went up to the roof/ceiling area

The only common exception is when el is part of a proper name:

• Voy a El Escorial
  not al Escorial

Because subir expresses movement upward toward a destination.

So:

• subir al techo = to go up to the ceiling / toward the ceiling

If you said en el techo, that would describe location:

• están en el techo = they are on the ceiling

With balloons, Spanish often uses subir al techo to mean they rise upward until they reach the ceiling area.

So the difference is basically:

• al techo = movement toward it
• en el techo = position on it/at it

Yes, subir is perfectly natural here.

Subir can mean:

• to go up
• to rise
• to move upward

For balloons, it means they float upward or rise.

So:

• los globos subieron = the balloons went up / rose
• para que no subieran al techo = so that they wouldn’t rise to the ceiling

In English, rise, go up, and float up could all fit depending on context. Spanish simply uses subir very naturally here.

Spanish often omits subject pronouns when the context is clear.

In this sentence, the most logical subject of subieran is los globos, because they are the thing that can physically rise to the ceiling:

• Dejé los globos... para que no subieran...

Spanish speakers normally rely on context for this. You could make it explicit, but it would usually be unnecessary:

• Dejé los globos junto a la silla para que ellos no subieran al techo.

That version is grammatical, but ellos sounds unnecessary unless you are adding contrast or emphasis.

Yes. Subieran and subiesen are both valid forms of the imperfect subjunctive.

So both are correct:

• para que no subieran al techo
• para que no subiesen al techo

In modern usage, -ra forms like subieran are generally more common, but -se forms are also fully correct and understood, including in Spain.

Yes. In context, dejar often means not just to leave, but also to leave something somewhere, which can sound like to put or to set down in English.

So:

• Dejé los globos junto a la silla could be understood as
  • I left the balloons next to the chair
  • I put the balloons next to the chair

The exact English translation depends on style and context, but the Spanish is very natural either way.