Forramos el cartón gris con la tela de encuadernar.

Forramos is the 1st person plural form of forrar, meaning to cover, to line, or to wrap something.

It can be grammatically ambiguous in isolation:

• present tense: we cover / we line
• preterite: we covered / we lined

In craft instructions, Spanish often uses the present tense to describe steps, so here it may mean something like we cover the grey cardboard with the bookbinding cloth. But without more context, both readings are possible.

In Spanish, adjectives usually come after the noun, so:

• cartón gris = grey cardboard

That is the normal word order. Putting gris before the noun would sound unusual here and would not be the standard way to describe the material.

Also, gris does not change for gender in this use:

• el cartón gris
• la tela gris

Cartón usually means cardboard, pasteboard, or board.

In a bookbinding or craft context, cartón gris often refers to the kind of grey board used for covers or backing. So it is not necessarily the same as thin packaging cardboard; it can mean a thicker, stiffer board depending on context.

Because the nouns have different grammatical genders:

• el cartón = masculine
• la tela = feminine

In Spanish, articles must match the gender and number of the noun:

• el with singular masculine nouns
• la with singular feminine nouns

These genders are grammatical, so they do not always correspond to anything logical in English. They simply have to be learned with the noun.

Con means with.

Here it introduces the material or thing used to cover the cardboard:

• Forramos el cartón gris con la tela de encuadernar.
• We cover the grey cardboard with the bookbinding cloth.

So con tells you what is being used to do the covering.

Literally, it means something like cloth for binding or bookbinding cloth.

Breakdown:

• tela = cloth / fabric
• de encuadernar = for binding / used for binding

This is a common Spanish structure:

• noun + de + infinitive

It often expresses purpose or intended use.

Similar patterns:

• máquina de coser = sewing machine
• papel de envolver = wrapping paper
• gafas de leer = reading glasses

So tela de encuadernar is cloth intended for bookbinding.

Because after de in expressions like this, Spanish often uses the infinitive to show purpose or function.

So:

• tela de encuadernar = cloth for binding
• not a fully conjugated verb, but an infinitive describing what the cloth is used for

This is very natural in Spanish and often corresponds to English forms like:

• for + -ing
• an adjective
• a compound noun

Spanish uses definite articles more often than English does.

In this sentence, the speaker is probably referring to specific materials involved in the task:

• el cartón gris = the grey cardboard
• la tela de encuadernar = the bookbinding cloth

Even where English might sometimes say just grey cardboard or bookbinding cloth, Spanish often prefers the article when talking about the specific item being handled in the process.

The most natural choice with forrar is usually forrar algo con algo:

• forrar el cartón con tela

So con is the standard preposition here.

Spanish does sometimes use other structures in other contexts, but for this sentence, con la tela de encuadernar is the safest and most natural option.

They are similar, but not always identical.

• cubrir = to cover
• forrar = to cover/line/wrap, often by putting a layer of material over a surface

In a craft or bookbinding context, forrar is especially appropriate because it suggests covering something neatly with a material, such as cloth, paper, or plastic.

So forrar is more specific and more natural than cubrir here.

The rr in forramos is the strong rolled r sound used in Spanish.

A rough guide:

• fo-rra-mos
• the rr is trilled more strongly than a single r

This strong rr sound appears when written as rr between vowels, as in:

• perro
• forrar
• guitarra

That makes forramos different in pronunciation from a word with a single r between vowels.

In standard Spanish, gris usually stays the same in both masculine and feminine, singular and plural, except that the plural adds -es:

• cartón gris
• tela gris
• cartones grises
• telas grises

So in your sentence, gris is singular because cartón is singular.

Yes, very possibly.

Spanish often uses the 1st person plural in instructions, recipes, tutorials, and demonstrations:

• Cortamos...
• Pegamos...
• Forramos...

This has the sense of we now do this step, and in English it is often translated more naturally as:

• Now cover the grey cardboard with the bookbinding cloth
• or We cover the grey cardboard with the bookbinding cloth

So the sentence fits very well in a step-by-step instructional context.