Guardamos el magnesio en la bolsa de magnesio.

Guardamos is the 1st person plural form of guardar, so it means we keep / we store / we put away.

In this sentence, it is in the present indicative:

• yo guardo = I keep / store
• tú guardas = you keep / store
• él/ella guarda = he/she keeps / stores
• nosotros guardamos = we keep / store
• vosotros guardáis = you all keep / store
• ellos guardan = they keep / store

Depending on context, guardar can mean:

• to keep
• to store
• to put away
• to save

Here, put away or store is probably the most natural sense.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

So:

• Guardamos el magnesio... = We store/put away the chalk...
• Nosotros guardamos el magnesio... = also correct, but more explicit

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

• Nosotros guardamos el magnesio, pero ellos guardan las cuerdas. = We put away the chalk, but they put away the ropes.

Spanish often uses the definite article with nouns where English would not.

So:

• el magnesio = literally the magnesium/chalk
• but in natural English this may simply be magnesium or chalk

This is very common with:

• materials
• abstract nouns
• general categories
• things already understood in the situation

For example:

• Me gusta el café. = I like coffee.
• Compramos el pan. = We buy bread.

So el magnesio does not necessarily mean a very specific individual piece of magnesium; it can just be the substance being referred to in context.

Because en is the normal preposition for in / inside.

• en la bolsa = in the bag

If you say a la bolsa, that sounds more like to the bag, focusing on direction rather than location. After verbs like guardar, Spanish usually uses en when something ends up inside a container:

• Guardar algo en una caja = to put/store something in a box
• Guardar algo en un cajón = to put/store something in a drawer

So Guardamos el magnesio en la bolsa... is the expected structure.

Because de + noun is often used in Spanish to describe type, purpose, or function.

So:

• bolsa de magnesio = chalk bag / magnesium bag
  • literally: bag of chalk/magnesium
  • more naturally: bag for chalk

This is similar to:

• taza de café = coffee cup
• zapatos de deporte = sports shoes
• gafas de sol = sunglasses

If you said la bolsa del magnesio, that would sound more like:

• the bag belonging to the magnesium or
• the bag of the magnesium already mentioned

That is not the intended meaning here. The sentence means a bag whose purpose is to hold chalk/magnesium.

Yes, very often, especially in sports contexts such as climbing or gymnastics, magnesio is used to refer to chalk.

So although the word literally corresponds to magnesium, in real usage it often means the white powder used on the hands.

That is why:

• bolsa de magnesio is a standard way to say chalk bag

This is one of those cases where the everyday sports meaning matters more than the strict chemical meaning.

It is grammatical, but it can sound a bit repetitive because magnesio appears twice:

• Guardamos el magnesio en la bolsa de magnesio.

A native speaker might often shorten it if the context is clear:

• Guardamos el magnesio en la bolsa.
• Metemos el magnesio en la bolsa de magnesio.

That said, the full version is still useful for learners because it clearly shows the vocabulary.

All three are possible in different contexts, but they are not exactly the same.

• guardar = to put away / store / keep
  • focuses on putting something in its proper place
• poner = to put / place
  • more general
• meter = to put in / stick in
  • focuses on movement into a space or container

So:

• Guardamos el magnesio en la bolsa = we put the chalk away / store it in the bag
• Ponemos el magnesio en la bolsa = we put the chalk in the bag
• Metemos el magnesio en la bolsa = we put the chalk into the bag

In your sentence, guardar suggests tidying it away or storing it.

Yes, in the right context, guardamos can sometimes be understood as a suggestion, especially in spoken Spanish with question intonation:

• ¿Guardamos el magnesio en la bolsa? = Shall we put the chalk in the bag?

But without question marks, as in your sentence, it is normally read as a statement:

• We put away / store the chalk in the chalk bag.

So punctuation and intonation matter.

Because bolsa is a feminine noun in Spanish, so it takes:

• la bolsa
• una bolsa

Adjectives and articles must agree with it:

• la bolsa pequeña
• una bolsa grande

This is something you usually just have to learn with the noun.

In standard Peninsular Spanish, it is pronounced approximately:

mag-NE-syo

More carefully:

• mag as in magnitude (but with a Spanish g sound)
• ne
• sio sounds roughly like syo

A broad IPA version is /maɣˈnesjo/.

Also, in Spain:

• g before a is a hard g
• there is no English j sound here

And bolsa is approximately:

• BOL-sa

After de, Spanish often leaves out the article when the noun is being used to classify or describe something.

So:

• bolsa de magnesio = a bag of/for chalk
• mesa de madera = wooden table
• botella de agua = bottle of water

Adding the article would usually change the meaning and make it more specific:

• la bolsa del magnesio = the bag of the magnesium / the bag that belongs to the magnesium already mentioned

Without the article, it works like a label or category: chalk bag.

Yes, in many contexts Spanish uses en both for location and for the endpoint after certain verbs.

So:

• Está en la bolsa. = It is in the bag.
• Guardamos el magnesio en la bolsa. = We put/store the chalk in the bag.

English often separates these ideas more clearly:

• in = location
• into = movement

Spanish often relies on the verb itself to show whether the meaning is static or dynamic. Here, guardamos already implies movement into the bag or storing inside it.