Kunt u op de achterkant van de envelop ook de naam van de afzender schrijven?

Because this is a question.

• U kunt = You can
• Kunt u ... ? = Can you ... ?

In Dutch yes/no questions, the verb usually comes before the subject:

• U schrijft de naam. = You write the name.
• Schrijft u de naam? = Do you write the name?

So:

• U kunt ... = You can ...
• Kunt u ... ? = Can you ... ?

Also, Kunt u ... ? is a polite way to make a request, similar to Could you ... ? in English.

U is the formal/polite word for you.

This sentence uses u because it is polite:

• Kunt u ... ? = Can you ... ? / Could you ... ?

If you were speaking informally to a friend, you might say:

• Kun je ... ?
• Kun jij ... ?

So:

• u = formal/polite you
• je/jij = informal you

Because the verb kunnen changes depending on the subject.

Present tense of kunnen:

• ik kan
• u kunt / kan
• jij kunt
• hij/zij/het kan
• wij kunnen
• jullie kunnen
• zij kunnen

With u, both kunt and kan are possible in modern Dutch, but kunt u is very common in polite questions.

So:

• Kunt u ... ? = Can you ... ?
• Kan u ... ? is also heard, especially in Belgium, but learners will most often meet kunt u in standard teaching materials.

It means on the back of the envelope.

Breakdown:

• op = on
• de achterkant = the back / backside
• van = of
• de envelop = the envelope

So literally:

• op de achterkant van de envelop = on the backside of the envelope

Dutch often uses van where English uses 's or an of phrase.

Because achter usually means behind or at the back, while achterkant is a noun meaning back side or rear side.

Compare:

• achter het huis = behind the house
• de achterkant van het huis = the back of the house

So in this sentence:

• de achterkant van de envelop = the back side of the envelope

You need the noun achterkant because you are naming a part of the envelope.

Dutch nouns have common gender (de) or neuter gender (het), and envelop is a de-word:

• de envelop

Unfortunately, gender usually has to be learned with each noun.

Other nouns in this sentence are also de-words:

• de achterkant
• de naam
• de afzender

So the sentence contains only de nouns.

Ook means also or too.

In this sentence:

• Kunt u ... ook de naam van de afzender schrijven?
• = Can you also write the sender’s name ... ?

Here ook adds the idea that this is an additional thing to write.

Its position can vary depending on emphasis, but Dutch word order with adverbs can be tricky. In this sentence, ook appears before the noun phrase de naam van de afzender, which sounds natural.

Very roughly:

• ook often comes before the part that is being added or emphasized.

So here the added item is:

• de naam van de afzender = the sender’s name

Because Dutch often sends the main infinitive to the end when there is another verb like kunnen.

Here:

• kunt = finite verb
• schrijven = infinitive

This is a very common Dutch pattern:

• Ik kan zwemmen. = I can swim.
• U kunt hier wachten. = You can wait here.
• Kunt u de naam schrijven? = Can you write the name?

So in the sentence:

• Kunt u ... schrijven?

the conjugated verb kunt comes near the front, and the infinitive schrijven goes to the end.

Afzender means sender.

It comes from the verb afzenden, which means to send off / dispatch.

A useful pattern in Dutch is:

• verb stem + -er = a person who does the action

Examples:

• werken → werker = worker
• lezen → lezer = reader
• afzenden → afzender = sender

So:

• de naam van de afzender = the name of the sender

Dutch usually prefers the van construction in many everyday cases:

• de naam van de afzender = the name of the sender

A form like de afzenders naam is possible in some contexts, but it is less neutral here and can sound more formal or stylistically marked.

For learners, van is the safest and most common choice:

• de kleur van de auto = the color of the car
• de deur van het huis = the door of the house
• de naam van de afzender = the sender’s name

In practice, it is usually a polite request, not a literal question about ability.

So although it literally looks like:

• Can you ... ?

it often really means:

• Could you ... ?
• Would you please ... ?

This is very common in Dutch, just like in English.

If someone asks:

• Kunt u op de achterkant van de envelop ook de naam van de afzender schrijven?

they are normally asking you to do it, not wondering whether you are physically capable of writing.

Yes, but it sounds a bit more formal or official.

Compare:

• op de achterkant van de envelop = on the back of the envelope
• op de achterzijde van de envelop = on the reverse side / back side of the envelope

Achterkant is very common and natural in everyday speech. Achterzijde is more formal, written, or official-sounding.

Not very naturally. A word-for-word version would be something like:

• Can you on the back side of the envelope also the name of the sender write?

That shows the Dutch structure, but it is not natural English.

A better natural English translation would be:

• Could you also write the sender’s name on the back of the envelope?

This is a good reminder that Dutch and English often use different word order, especially with:

• questions
• modal verbs like kunnen
• infinitives at the end
• van phrases such as de naam van de afzender