De envelop ligt al klaar op tafel, maar zonder adres kan ik hem niet versturen.

Because envelop is a de-word in Dutch. That means:

• singular: de envelop
• plural: de enveloppen

This also matters later in the sentence, because the pronoun referring back to it is hem.

Dutch often uses position verbs where English would simply use to be.

Here, liggen means to lie and is used for things that are lying flat or resting horizontally. An envelope on a table is naturally thought of as lying there, so ligt sounds very normal.

So:

• De envelop ligt op tafel = the envelope is lying on the table
• De envelop is op tafel is much less natural here

Dutch does this a lot with:

• liggen for flat/horizontal things
• staan for upright things
• zitten for things located in/on/inside something in a less literal way

It is both understandable word by word and also a very common fixed combination.

klaarliggen means to be lying ready / to be prepared and waiting. In a main clause, Dutch splits this up because the finite verb goes in second position:

• De envelop ligt al klaar

In dictionary form or at the end of a clause, you may see klaarliggen.

So a learner can think of it as a useful expression: klaarliggen = to be ready and waiting.

Al means already here.

It gives the idea that the envelope is ready by now / beforehand / at this point.

So ligt al klaar suggests something like:

• it has already been prepared
• that part is already done

Both are possible, but they are not used in exactly the same way.

• op tafel is the more natural, idiomatic way to say on the table
• op de tafel sounds more specific, more contrastive, or more deliberately literal

In everyday Dutch, op tafel is very common when the table is just the obvious location in the situation.

Compare:

• Het ligt op tafel. = It’s on the table.
• Niet op de kast, maar op de tafel. = Not on the cupboard, but on the table.

So in your sentence, op tafel is the most natural choice.

After zonder, Dutch often uses a bare noun when speaking in a general sense.

So zonder adres means without an address in a broad, natural way.

This is similar to other Dutch expressions such as:

• zonder geld = without money
• zonder probleem = without a problem / no problem
• zonder toestemming = without permission

You can say zonder een adres, but that sounds more specific or more emphatic. In this sentence, zonder adres is the normal choice.

This is because Dutch main clauses follow the verb-second rule.

After maar, you start a new main clause. The first element in that new clause is zonder adres. Because that element comes first, the finite verb must come next:

• zonder adres → first element
• kan → finite verb in second position
• ik → subject
• hem → object
• niet
• versturen → infinitive at the end

So the structure is:

maar + zonder adres + kan + ik + hem + niet + versturen

If the clause began with ik, you would get:

• maar ik kan hem zonder adres niet versturen

That is also possible, but it has a slightly different emphasis.

Because hem refers back to de envelop, and envelop is a de-word.

In standard Dutch:

• a singular de-word often takes hem as object pronoun
• a singular het-word takes het

So:

• de envelop → hem
• het boek → het

In casual Dutch, some speakers may sometimes use a demonstrative like die, but hem is the standard personal pronoun here.

Because the sentence has a modal verb:

• kan = can
• versturen = send

In Dutch, with a modal + infinitive structure, niet usually comes before the infinitive when it negates the action:

• ik kan hem niet versturen = I cannot send it

So the idea is that the sending is not possible.

This is a very common pattern:

• Ik kan niet komen.
• Ze wil het niet doen.
• Wij mogen daar niet parkeren.

Yes, there is some difference in tone and usage.

• versturen = to send off; very natural for letters, emails, messages, parcels
• sturen = to send; more general
• opsturen = to send off / send in / send along, often with a sense of direction toward someone or somewhere
• verzenden = to dispatch; more formal or business-like

For an envelope, versturen sounds very natural and everyday.

Because maar joins two main clauses here, and in Dutch that is normally separated with a comma.

The sentence has two parts:

• De envelop ligt al klaar op tafel
• maar zonder adres kan ik hem niet versturen

Putting a comma before maar makes the structure clear and is standard punctuation.

You could, but it would not sound as natural in this context.

is klaar focuses only on the idea of being ready.
ligt klaar sounds more Dutch here because it combines:

• the physical position of the envelope
• the idea that it is ready and waiting

Since the envelope is actually lying on the table, ligt al klaar is the best choice.