Mi hermana puso la planta en la balda más baja para que le diera menos sol.

Balda means shelf, especially a flat shelf board attached to a wall or inside a shelving unit.

In Spain, balda is a very normal word for this. In some other varieties of Spanish, learners may hear estante more often, but in Spain balda is very common.

Puso is the preterite of poner, so it presents the action as a completed event:

• Mi hermana puso la planta... = she placed it / put it there

If you used ponía, that would be the imperfect, which usually suggests:

• a repeated action
• a background action
• an ongoing past action

So here puso fits because the sentence describes one specific action: she moved the plant to that shelf.

Because balda is a feminine singular noun, the adjective has to agree with it:

• la balda
• baja

And más baja means lower / lowest, depending on context.

So:

• la balda más baja = the lowest shelf

This is a very common way to express the superlative in Spanish:

• el libro más interesante = the most interesting book
• la mesa más grande = the biggest table

Because para que is used when a new subject appears in the purpose clause.

Here the main clause is about mi hermana, but the second part is about the sun affecting the plant, not about the sister doing something directly.

• Mi hermana puso la planta...
• para que le diera menos sol

A useful rule:

• para + infinitive → usually when the subject stays the same
• para que + subjunctive → when there is a different subject or a full new clause

Compare:

• Moví la planta para protegerla.
  Same subject: I moved / I protect
• Moví la planta para que recibiera menos sol.
  Full clause after para que

Because para que normally requires the subjunctive.

Since the main verb is in the past (puso), Spanish uses the imperfect subjunctive in the purpose clause:

• puso ... para que diera

This is a very common pattern:

• Lo hice para que funcionara.
• Cerró la ventana para que no entrara frío.

If the main verb were in the present, you would normally use the present subjunctive:

• Pone la planta ahí para que le dé menos sol.

So:

• past main verb → often imperfect subjunctive
• present/future main verb → often present subjunctive

Le refers to la planta.

It is an indirect object pronoun here. With dar, Spanish often marks the thing receiving something as an indirect object:

• darle sol a la planta
• el sol le da a la planta

So in this sentence:

• para que le diera menos sol
  = so that the plant would get less sun / so that less sun would hit it

Even though English might use it, Spanish uses le because of how dar works.

Because dar el sol or dar sol is a very natural Spanish way to talk about sunlight reaching something.

Examples:

• Aquí da mucho sol. = This spot gets a lot of sun.
• A esta habitación le da el sol por la mañana. = This room gets sun in the morning.
• A la planta le daba demasiado sol. = The plant was getting too much sun.

In this sentence, sol is being treated a bit like sunlight as an amount, which is why menos sol sounds very natural.

Because when menos directly modifies a noun, Spanish normally does not use de:

• menos agua
• menos tiempo
• menos ruido
• menos sol

You usually see menos de before a number or a more specific quantified expression:

• menos de diez minutos
• menos de dos euros

So here:

• menos sol = less sun / less sunlight

Sometimes yes, but they are not always exactly the same.

In Spain:

• balda usually means the individual shelf board
• estante can mean a shelf too, but it can also refer more broadly to shelving or a shelf space

So balda is a very good choice here if you mean the actual shelf level where the plant was placed.

Spanish more naturally says para que le diera menos sol here.

That is because sol is being used almost like an uncountable noun meaning sunlight:

• Aquí da mucho sol
• En esa esquina da menos sol

You will also hear darle el sol in other contexts:

• No pongas la planta donde le dé el sol directo.

So both patterns exist, but in this sentence menos sol is the most natural wording.

Because Spanish normally keeps the object pronoun with verbs like dar, even when the noun has already appeared earlier in the sentence.

So Spanish likes:

• Puso la planta donde le diera menos sol
• Le dio agua a la planta
• No le da suficiente luz

This is very normal and often sounds more natural than omitting the pronoun.