Ich sende den Erstattungsantrag heute.

Because Erstattungsantrag is the direct object of the verb senden. In German, a masculine noun changes from der in the nominative to den in the accusative.

• der Erstattungsantrag = subject form
• den Erstattungsantrag = direct-object form

Here, ich is the subject and den Erstattungsantrag is the thing being sent, so the accusative is needed.

You have to learn the noun together with its article: der Erstattungsantrag.

There is no completely reliable rule from the ending alone here, so the safest way is to memorize:

• der Antrag = application/request
• der Erstattungsantrag = refund request / reimbursement claim

A useful tip: in compound nouns, the last part determines the gender. Since Antrag is masculine (der Antrag), Erstattungsantrag is also masculine.

German main clauses usually follow the verb-second rule.

In this sentence:

• Ich = first position
• sende = second position
• den Erstattungsantrag heute = the rest of the sentence

So Ich sende ... is the normal German structure for a statement.

Because the verb has to be conjugated to match ich.

The infinitive is senden.
The ich form is ich sende.

Present tense of senden:

• ich sende
• du sendest
• er/sie/es sendet
• wir senden
• ihr sendet
• sie/Sie senden

So Ich sende den Erstattungsantrag heute is grammatically correct because the verb agrees with ich.

Yes. German word order is flexible, as long as the conjugated verb stays in the correct position in a main clause.

These are all possible, with slightly different emphasis:

• Ich sende den Erstattungsantrag heute.
• Ich sende heute den Erstattungsantrag.
• Heute sende ich den Erstattungsantrag.

If you put heute first, then the verb must still stay in second position:

• Heute sende ich ...
• not Heute ich sende ...

In German, the present tense often covers both present and near future, especially when there is a time word like heute.

So this sentence can mean something like:

• I am sending the reimbursement request today
• I will send the reimbursement request today

German often uses the simple present where English might prefer will or be ... -ing.

It is a compound noun, which is very common in German.

It is made up of:

• Erstattung = reimbursement / refund
• Antrag = application / request

Together:

• Erstattungsantrag = reimbursement application / refund request / reimbursement claim

German frequently combines nouns into one long word instead of writing several separate words.

They are similar, but not always identical.

• schicken is very common in everyday speech and often means to send
• senden can sound a bit more formal or technical in some contexts

In an administrative or business sentence like this one, senden fits well.

For example:

• Ich schicke den Brief heute. = everyday, natural
• Ich sende den Erstattungsantrag heute. = also natural, slightly more formal or official-sounding

Because German does not need a marker like English to in this structure.

In English, you say:

• I send the reimbursement request today

German works similarly here:

• Ich sende den Erstattungsantrag heute

The object is shown by case (den), not by a preposition.
And heute is just an adverb meaning today, so it does not need a preposition either.

Yes, that is possible, and it can sound very natural.

The separable verb absenden means to send off / submit.

So you could say:

• Ich sende den Erstattungsantrag heute ab.
• Ich sende heute den Erstattungsantrag ab.

This often emphasizes the act of actually sending or submitting the document.

By contrast:

• senden = send
• absenden = send off / dispatch / submit

Both can work, depending on context.

Then the article would change.

For example:

• singular: Ich sende den Erstattungsantrag heute.
• plural: Ich sende die Erstattungsanträge heute.

Why?

• masculine accusative singular: den
• plural accusative: die

Also note the plural form of the noun:

• der Erstattungsantrag
• die Erstattungsanträge

So the article and noun ending both change in the plural.