Mi madre dice que el sello está dentro del paquete, debajo del libro.

Because mi without an accent is the possessive adjective meaning my.

• mi madre = my mother
• mí with an accent is a pronoun used after prepositions:
  • para mí = for me
  • a mí = to me

So in this sentence, mi madre is correct because you are describing your mother.

Dice is the present tense of decir for él/ella/usted.

• mi madre dice = my mother says
• dijo = said in the past
• diga = a subjunctive form, used in very different contexts
• está diciendo = is saying, which is possible in some situations, but less neutral here

Spanish often uses the simple present where English might also use the simple present, so mi madre dice is the normal choice.

Because que introduces the clause that tells you what she says.

• Mi madre dice que... = My mother says that...

This works very much like English that, although in Spanish que is used very regularly after verbs like decir, pensar, creer, and saber.

Examples:

• Dice que viene. = She says that she is coming.
• Creo que sí. = I think so / I think that yes.

Sello is a masculine noun, so it takes el in the singular.

• el sello
• los sellos

A useful pattern is that many nouns ending in -o are masculine, although there are exceptions.

Also, depending on context, sello can mean:

• stamp as in a postage stamp
• seal as in an official seal or stamp mark

So the exact meaning depends on the situation.

Because Spanish uses estar for location.

• El sello está dentro del paquete. = The stamp/seal is inside the package.

This is one of the most important uses of estar:

• Madrid está en España.
• El libro está en la mesa.

Ser is generally not used for physical location, except in special cases such as events:

• La fiesta es en mi casa.

So here está is correct because the sentence is about where the object is.

Both can be correct, but they are slightly different in emphasis.

• en el paquete = in the package
• dentro del paquete = inside the package

Dentro de sounds a bit more explicit about being physically inside something. It often feels slightly more precise or emphatic.

So:

• El sello está en el paquete is fine.
• El sello está dentro del paquete stresses the idea of being inside.

Because del is the contraction of de + el.

• dentro de el paquete becomes dentro del paquete
• debajo de el libro becomes debajo del libro

This contraction is mandatory in standard Spanish whenever de is followed by the masculine singular article el, unless El is part of a proper name.

Compare:

• del libro = de + el libro
• de la mesa = no contraction, because it is la
• de El Escorial = no contraction, because El is part of the name

Both can mean under the book, but debajo de is often more common and more clearly locational.

• debajo del libro = underneath the book
• bajo el libro = also possible, but sometimes sounds a bit more formal or literary depending on context

For everyday physical location, learners will hear debajo de very often.

The comma separates two location phrases that add information about where the object is.

The sentence gives a more precise location in two steps:

• dentro del paquete = inside the package
• debajo del libro = under the book

So the full idea is that the stamp/seal is:

• inside the package
• and specifically under the book

The comma helps the sentence flow, but in some contexts people might also say it without a comma, especially in speech.

Yes. Spanish often combines several place expressions in one sentence to narrow down the location.

For example:

• Está en la caja, al lado de la llave.
• Está en la habitación, encima de la mesa.
• Está dentro del paquete, debajo del libro.

This is completely natural. Each phrase adds another layer of detail.

In modern standard Spanish, you normally use just the possessive:

• mi madre
• tu hermano
• su casa

Using la mi madre is not standard in modern general Spanish. You may see similar structures in older language or in some regional varieties, but for normal modern usage, mi madre is the correct form.

In this sentence, the subject is expressed because the speaker wants to say my mother says...

But once the subject is clear in context, Spanish can often omit subject pronouns like ella:

• Mi madre dice que...
• Dice que... if it is already obvious who is speaking

Spanish does this a lot because the verb form already gives information about the subject. However, here mi madre is useful because it identifies exactly who says it.

Because this sentence uses a very common and neutral Spanish structure:

• subject: Mi madre
• verb: dice
• subordinate clause: que el sello está...

That pattern is extremely common in Spanish:

• Mi amigo cree que...
• El profesor dice que...
• Ana piensa que...

The second part also follows a natural order:

• thing being located: el sello
• verb of location: está
• place phrases: dentro del paquete, debajo del libro

So although Spanish and English do not always match word for word, this sentence happens to line up fairly closely.

In Spain, a natural pronunciation would be roughly:

Mi madre dice que el sello está dentro del paquete, debajo del libro.

A few pronunciation points:

• dice: the c before e is pronounced like the th in thin in most of Spain
• sello: the ll is usually pronounced like a y sound in modern Spain
• está: the final syllable is stressed because of the written accent
• paquete: stress falls on que

So in broad terms, someone from most of Spain might say something like: Mi madre dithe que el seyo está dentro del pakete, debajo del libro.

That spelling is only an approximation to help an English speaker hear it.