Pongo la cáscara del huevo en el cubo de la basura.

Because poner is irregular in the yo form of the present tense.

• poner = to put
• yo pongo = I put / I am putting

Many common Spanish verbs have an irregular yo form, and poner is one of them:

• yo pongo
• tú pones
• él/ella pone

So pongo is the correct form here.

Spanish often leaves out subject pronouns when they are clear from the verb ending.

• Pongo already means I put or I am putting
• So yo pongo is possible, but yo is usually unnecessary

You would add yo only if you want emphasis or contrast:

• Yo pongo la cáscara, tú limpias la mesa.
  = I put the shell in the bin, you clean the table.

It is the present indicative.

In Spanish, the present can mean different things depending on context:

• I put
• I am putting
• I usually put

So Pongo la cáscara del huevo en el cubo de la basura could mean:

• I’m putting the eggshell in the bin right now
• I put the eggshell in the bin
• I usually put the eggshell in the bin

The wider context tells you which meaning is intended.

Spanish uses articles more often than English does.

Here’s how they work in this sentence:

• la cáscara = the shell
• del huevo = of the egg
• el cubo = the bin / bucket
• de la basura = of the rubbish / trash

In natural Spanish, those articles sound normal. English often drops them where Spanish keeps them.

So even if English might say something shorter like I put eggshell in the trash, Spanish usually prefers the fuller structure.

Because de + el contracts to del in Spanish.

• de el huevo → del huevo

This contraction is mandatory in normal Spanish.

The main exception is when El is part of a proper name:

• de El Escorial
• de El Salvador

But in your sentence, del huevo is the normal and correct form.

These two expressions are related, but they are not exactly the same.

• la cáscara del huevo = the shell of the egg / the eggshell from this specific egg
• cáscara de huevo = eggshell, in a more general or type-of-material sense

In your sentence, la cáscara del huevo sounds like a specific shell from a specific egg, which fits the situation very well.

Compare:

• Tiro la cáscara del huevo.
  = I throw away the shell of the egg / this eggshell.
• La cáscara de huevo sirve como abono.
  = Eggshell is useful as fertiliser.

Cáscara means the outer covering of something, especially when it is hard or peel-like.

Examples:

• la cáscara del huevo = eggshell
• la cáscara de una naranja = orange peel
• la cáscara de una nuez = nutshell

For an egg, cáscara is the normal word for the shell.

You may also see cascarón, but that often refers more to the shell as a hollow shell, or can have other specific uses depending on context. For a simple sentence like this, cáscara is the most straightforward choice.

With poner, Spanish normally uses en to show where something is placed.

• poner algo en algo = to put something in/on something

So:

• Pongo la cáscara en el cubo = I put the shell in the bin

Using a here would not be correct with poner in this structure.

A useful pattern to remember is:

• poner + object + en + place/container

Yes, it is especially common in Spain.

In Spain:

• el cubo de la basura = the rubbish bin / trash bin

Literally, cubo means bucket, but in everyday Spanish from Spain it is a normal way to refer to the household trash bin.

In other Spanish-speaking countries, other words may be more common, such as:

• basurero
• bote de basura
• tacho de basura

So your sentence sounds very natural for Spanish from Spain.

Often, yes.

Your sentence is grammatical and understandable, but if the idea is specifically throwing something away, many speakers would more naturally say:

• Tiro la cáscara del huevo al cubo de la basura.

Why?

• poner focuses on placing something
• tirar focuses on throwing something away

So:

• Pongo la cáscara del huevo en el cubo de la basura = I put the eggshell in the bin
• Tiro la cáscara del huevo al cubo de la basura = I throw the eggshell in the bin

Both work, but tirar may sound more everyday in this context.

Because the stress falls on the first syllable: CÁS-ca-ra.

Without the accent mark, Spanish spelling rules would make you stress the second-to-last syllable:

• cascara would naturally sound like cas-CA-ra

But the real pronunciation is CÁS-ca-ra, so the written accent is needed.

Yes, Spanish word order is more flexible than English word order, although the original sentence is the most neutral one.

Neutral order:

• Pongo la cáscara del huevo en el cubo de la basura.

Possible variations for emphasis:

• La cáscara del huevo la pongo en el cubo de la basura.
  This emphasises the eggshell.
• En el cubo de la basura pongo la cáscara del huevo.
  This emphasises where it goes.

So the original version is the standard, natural choice, but other orders are possible depending on emphasis.

Yes. That is very common.

• Tiro la cáscara del huevo a la basura.
  = I throw the eggshell in the trash.

This version is shorter and very natural. It does not mention the physical bin explicitly; it just means you are throwing it away.

So there is a small difference:

• en el cubo de la basura = into the rubbish bin
• a la basura = in the trash / away

Both are correct, but a la basura is more idiomatic when the exact container does not matter.