La guarda cubre el cartón gris.

Here, guarda is a noun. You can tell because it is preceded by the article la: la guarda.

That matters because guarda can also be a verb form of guardar:

• él/ella guarda = he/she keeps
• guarda = keep! (informal singular command)

But in this sentence, the structure la + guarda shows that it is a noun phrase, not a verb.

Because the noun used here is feminine singular, so it takes la.

Spanish articles must agree with the noun:

• la
  • feminine singular noun
• el
  • masculine singular noun

A useful warning: guarda can also appear in other meanings where you may see el guarda or la guarda referring to a person. That is a different use. In this sentence, the feminine article la tells you which noun you are dealing with.

Cubre is the 3rd person singular present indicative of cubrir.

So:

• cubrir = infinitive
• cubre = he/she/it covers or you cover (formal usted)

It matches the singular subject la guarda.

A quick mini-conjugation:

• yo cubro
• tú cubres
• él/ella/usted cubre
• nosotros cubrimos
• vosotros cubrís
• ellos/ellas/ustedes cubren

Because el cartón gris is the direct object and it is presented as a specific piece of grey cardboard.

So the sentence has:

• La guarda = subject
• cubre = verb
• el cartón gris = direct object

Using el here is completely normal. It works like the in English.

Because cartón gris is an inanimate thing.

Spanish often uses the personal a before a specific person or sometimes an animal:

• Veo a María
• Visito a mi abuelo

But you normally do not use that a with things:

• cubre el cartón gris ✔
• cubre al cartón gris ✘ in normal usage

Because in Spanish, adjectives usually come after the noun, especially colour adjectives.

So the normal order is:

• cartón gris
• coche rojo
• casa blanca

Putting gris before the noun would sound marked, literary, or unnatural in a plain sentence like this.

For gender, no: gris stays the same.

• el cartón gris
• la caja gris

For number, yes: in the plural it becomes grises.

• los cartones grises
• las cajas grises

So gris is unchanged in singular masculine and singular feminine, but it changes in the plural.

The accent mark shows that the stress falls on the last syllable: car-TÓN.

That is important because Spanish stress follows default rules:

• words ending in a vowel, n, or s are normally stressed on the second-to-last syllable
• if the stress breaks that rule, a written accent is needed

So without the accent, carton would be read with the stress in the wrong place.

A simple English-friendly guide is:

la GWAR-da KOO-breh el kar-TON grees

A few details:

• gua in guarda sounds like gwa
• c in cubre and cartón is a hard k
• the r in cubre and cartón is a single tap, not a strong English r
• cartón is stressed on the last syllable because of the accent mark

A more phonetic version is:

[la ˈɡwaɾða ˈkuβɾe el kaɾˈton ɡɾis]

Yes, Spanish word order is more flexible than English, but the neutral, most natural order here is:

La guarda cubre el cartón gris.

That is:

• subject
• verb
• object

Other orders are possible for emphasis, contrast, or style, but they would sound less neutral. So for a learner, this sentence is a good example of standard word order.

In this sentence, the most natural reading is simple present: covers.

Spanish present tense can sometimes also express:

• habitual actions
• general truths
• even near-future meaning in some contexts

But with this standalone sentence, a learner should read cubre simply as covers.