Mi amiga cogió una tiza azul para explicar la diferencia entre el singular y el plural.

Cogió is the third-person singular preterite form of coger.

• coger = to take, pick up, grab
• cogió = she took / she picked up

It is in the preterite because the sentence describes a completed action in the past: your friend picked up a piece of chalk in order to explain something.

A quick breakdown:

• yo cogí = I took
• tú cogiste = you took
• él/ella cogió = he/she took

So Mi amiga cogió... means My friend took/picked up...

Yes. In Spain, coger is a very common and completely normal verb meaning things like:

• to take
• to pick up
• to catch (for example, a bus)

So in Spain, cogió una tiza sounds perfectly natural.

However, this is useful cultural knowledge: in some Latin American countries, coger can have a vulgar sexual meaning. Because of that, many learners are warned about it. But since this sentence is specifically for Spanish from Spain, coger is absolutely fine here.

Because mi is the correct possessive adjective for both masculine and feminine singular nouns.

So you say:

• mi amigo = my male friend
• mi amiga = my female friend

The possessive mi does not change for gender. It only changes for number:

• mi amiga = my friend
• mis amigas = my friends

Learners sometimes expect something like mia amiga, but that is not how possessive adjectives work here. The form mía exists, but it is used differently, for example:

• La amiga es mía = The friend is mine

There are two things happening here:

1. una tiza
   Tiza means chalk or a piece of chalk, and it is a feminine singular noun, so it takes una.

2. azul after the noun
   In Spanish, descriptive adjectives usually come after the noun:

   • una tiza azul = a blue piece of chalk

So the structure is:

• article + noun + adjective

Very typical Spanish word order.

Because azul is one of the adjectives that has the same form for masculine and feminine.

Compare:

• un libro azul = a blue book
• una tiza azul = a blue piece of chalk

It only changes for plural:

• libros azules
• tizas azules

So:

• singular: azul
• plural: azules

But there is no separate feminine singular form like azula.

Para explicar means in order to explain or simply to explain.

Here, para expresses purpose or intention. It tells you why she picked up the chalk.

• Mi amiga cogió una tiza azul para explicar...
• My friend picked up a blue piece of chalk to explain...

This is a very common pattern:

• Estudio para aprender. = I study in order to learn.
• Fui al supermercado para comprar leche. = I went to the supermarket to buy milk.

So para + infinitive is often used when the subject is doing something for the purpose of another action.

Because the sentence is referring to one specific difference: the difference between singular and plural.

So:

• la diferencia entre X y Y = the difference between X and Y

This is the normal way to say it in Spanish.

If there were multiple distinctions or several points of contrast, then las diferencias could be possible. But here the idea is a single grammatical contrast:

• singular vs. plural

That is why la diferencia sounds natural.

Because singular and plural are being used here as nouns, not just adjectives.

Normally, they are adjectives:

• un sustantivo singular = a singular noun
• formas plurales = plural forms

But in this sentence, they mean:

• el singular = the singular form / the singular
• el plural = the plural form / the plural

Spanish often uses the definite article like this when talking about grammatical categories, abstract concepts, or whole classes of things.

So:

• entre el singular y el plural = between the singular and the plural

Yes, you could say:

• Ella cogió una tiza azul...

But in Spanish, subject pronouns are often left out because the verb ending already shows who the subject is.

Here, cogió already tells us it is:

• he
• she
• or you (formal)

Since the sentence already has Mi amiga, adding ella would usually be unnecessary.

Natural Spanish prefers:

• Mi amiga cogió una tiza azul...

rather than:

• Ella, mi amiga, cogió una tiza azul...

unless you want special emphasis or contrast.

The accent mark shows the correct stress and helps distinguish the pronunciation.

Cogió is pronounced roughly like:

• coh-HYOH

The accent tells you the stress falls on the last syllable.

This is also part of the standard spelling pattern for many preterite forms ending in:

• -ó for él/ella/usted
• -é for yo

For example:

• hablé
• habló
• comí
• comió
• cogí
• cogió

So the accent is not optional; it is a normal part of the correct written form.

Yes. Spanish often turns adjectives into nouns by using the definite article.

So:

• singular is normally an adjective
• el singular means the singular
• plural is normally an adjective
• el plural means the plural

This is very common in Spanish. Similar examples are:

• lo importante = the important thing / what is important
• los ricos = the rich
• el español = the Spanish language / Spanish

So in your sentence, el singular and el plural are perfectly normal noun-like uses.

The given word order is the most neutral and natural:

• Mi amiga cogió una tiza azul para explicar la diferencia entre el singular y el plural.

But Spanish word order is somewhat flexible. Some changes are possible, especially for emphasis. For example:

• Para explicar la diferencia entre el singular y el plural, mi amiga cogió una tiza azul.

This puts more focus on the purpose first.

What usually stays the same is the internal order of smaller chunks:

• una tiza azul sounds natural
• azul tiza is wrong
• la diferencia entre el singular y el plural is the normal grouping

So yes, some movement is possible, but the original sentence is the most straightforward version.

Because here explicar is followed by what is being explained, not the person it is explained to.

• explicar algo = to explain something
• explicar algo a alguien = to explain something to someone

In your sentence:

• explicar la diferencia = to explain the difference

There is no person mentioned, so no a is needed.

If you added a person, then you would say:

• Mi amiga cogió una tiza azul para explicar la diferencia a los estudiantes.
• My friend picked up a blue piece of chalk to explain the difference to the students.