Ich wäre Ihnen sehr dankbar, wenn Sie mir alle Unterlagen zusenden könnten

Question

Why is the zu + infinitive construction required in this sentence? We do have the modal verb and should be followed by the verb alone. My guess is that this sentence would be:

Ich wäre Ihnen sehr dankbar, wenn Sie mir alle Unterlagen senden könnten.

Answer 1

This is not an infinitive structure zu + Infinitive, it is just a plain infinitive: "zusenden" is not a form of "senden", it is a word on its own. It has roughly the same meaning, though.

"zusenden" especially stresses that you’re sending something to somebody as opposed to just sending it away. Just saying "wenn Sie mir alle Unterlagen senden könnten" even sounds a bit clumsy to me. It is just more common to use "zusenden" when speaking about sending something to somebody.

Answer 2

In addition to Idmean's completely satisfying answer:

Here is your zusenden in an infinitive+zu construction:

Bitte verzeihen Sie, dass ich vergaß, Ihnen das Paket rechtzeitig zuzusenden.

Note that the first zu is the zu that is part of the lexem, and the second zu is the zu that comes with that particular construction.

How can we tell? Compare:

Bitte vergiß nicht, das Paket bei der Post aufzugeben.

(from aufgeben)

Answer 3

The construction called »zu + Infinitiv« never is build as one word that starts with the prefix zu.

If the verb is separable, zu is an interfix inside the word:

Ich habe dich gebeten einzukaufen.
I asked you to do the shopping.

Ich habe dich gebeten Milch einzukaufen.
I asked you to do buy milk.

It the verb is not separable, »zu« is a separate word:

Ich habe dich gebeten, deine alten Bücher zu verkaufen.
I asked you to sell your old books.

If you find zu as a prefix of a verb, then this is a distinct verb, and zu is a part of that verb:

Wir müssen die Kleinteile zukaufen.
We must buy the consumables in addition.

In the example above zukaufen is an infitive, but it is not »zu + Infinitiv«. But you can build this too:

Wir sind nicht in der Lage, Kleinteile zuzukaufen.
We are not able to buy consumables in addition.

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used verbs:

• (etwas) einkaufen = to do the shopping (to buy something)
  separable: Ich kaufe (etwas) ein
• etwas verkaufen = to sell something
  not separable: Ich verkaufe etwas
• etwas zukaufen = to buy something in addition
  separable: Ich kaufe etwas zu

Answer 4

Here "zu" is just a prefix in the verb, as it is in other words: zurecht (Recht, rechtens), Zugabe (Gabe, geben), zumindest (mindestens), zusammen (sammeln, beisammen)...