Chúng ta tự làm phẳng bìa các tông.

Chúng ta means we in an inclusive sense, so it includes both the speaker and the listener.

• chúng ta = we, including you
• chúng tôi = we, not including you

So if someone says Chúng ta tự làm phẳng bìa các tông, they are involving the listener in the action.

Tự adds the idea of oneself / by oneself / ourselves.

In this sentence, it suggests that we do it ourselves, often with a sense like:

• without someone else doing it for us
• by our own effort
• personally

So:

• Chúng ta làm phẳng bìa các tông = We flatten the cardboard.
• Chúng ta tự làm phẳng bìa các tông = We flatten the cardboard ourselves.

It is written as two words, but it functions together as a verb phrase meaning to flatten.

Literally:

• làm = make, do
• phẳng = flat

So làm phẳng literally means make flat.

This is a very common pattern in Vietnamese:

• làm sạch = make clean, clean
• làm khô = make dry, dry
• làm nóng = make hot, heat up
• làm phẳng = make flat, flatten

Bìa các tông means cardboard.

A few notes:

• bìa can mean a stiff sheet, cover, or board-like material depending on context.
• các tông comes from a borrowed word related to carton.

In modern Vietnamese, you may also see:

• bìa carton
• thùng carton = cardboard box

So bìa các tông is understandable and correct, though some speakers may more often write or say bìa carton.

Vietnamese word order here is:

• Chúng ta = subject
• tự = adverb-like modifier
• làm phẳng = verb
• bìa các tông = object

So the basic structure is:

Subject + modifier + verb + object

That is why tự comes before the verb phrase làm phẳng.

Vietnamese does not use articles the way English does.

So a bare noun like bìa các tông can mean:

• cardboard
• the cardboard
• some cardboard

The exact meaning depends on context.

If you want to be more specific, Vietnamese can add other words, for example:

• tấm bìa các tông = a piece/sheet of cardboard
• miếng bìa các tông = a piece of cardboard
• những tấm bìa các tông = cardboard sheets / some pieces of cardboard

Yes, it is grammatical and understandable. However, depending on the situation, a native speaker might say it a bit differently.

For example, if talking about a specific sheet of cardboard, you might hear:

• Chúng ta tự làm phẳng tấm bìa các tông.

That can sound a little more concrete because tấm is a classifier for flat objects like sheets or panels.

Still, the original sentence is fine, especially if cardboard is being talked about in a general sense.

You can leave it out if you do not need the idea of ourselves.

Compare:

• Chúng ta làm phẳng bìa các tông. = We flatten the cardboard.
• Chúng ta tự làm phẳng bìa các tông. = We flatten the cardboard ourselves.

So tự is not required for grammar, but it changes the nuance.

A rough guide is:

• bìa: like bee-uh, with a falling tone on bì
• các: roughly gak or kahk, with a high checked tone in Northern Vietnamese
• tông: roughly tohng, with a level high tone

The biggest challenge is not the consonants, but the tones and the short final stop in các.

If you are learning Northern Vietnamese, pay attention to:

• bìa = falling tone on the first syllable
• các = sắc tone with a final -c
• tông = ngang tone

Not in this sentence.

Tự can sometimes appear in words related to self- or auto-, such as:

• tự động = automatic

But by itself in Chúng ta tự làm phẳng bìa các tông, tự clearly means ourselves / by ourselves, not automatically.

So the sentence is about people doing the action themselves, not about the cardboard flattening by itself or by machine.