Ich fülle das Rücksendeformular aus.

Because ausfüllen is a separable verb. Its full dictionary form is ausfüllen = to fill out.

In a normal main clause, German often splits separable verbs:

• Ich fülle das Rücksendeformular aus.

The conjugated part (fülle) stays in the usual verb position, and the prefix (aus) goes to the end.

Compare:

• Ich fülle das Formular aus.
• ..., weil ich das Formular ausfülle.
  In a subordinate clause, the verb is usually not split.

The dictionary form is ausfüllen.

So if you look this up in a dictionary, you would search for ausfüllen, not just fülle or aus.

Because fülle is the 1st person singular present tense form, matching ich.

The verb ausfüllen / füllen is conjugated like this in the present tense:

• ich fülle
• du füllst
• er/sie/es füllt
• wir füllen
• ihr füllt
• sie/Sie füllen

So Ich fülle ... aus means I fill out ...

It is in the accusative case because it is the direct object of the verb.

The sentence answers the question What am I filling out?
Answer: das Rücksendeformular

For a neuter noun like das Formular, the article is the same in nominative and accusative:

• nominative: das Formular
• accusative: das Formular

So even though it is accusative here, the form does not change.

Because Formular is a neuter noun in German: das Formular.

Also, in German compound nouns, the last part determines the grammatical gender.
So in Rücksendeformular, the final word is Formular, which is neuter, so the whole compound is:

• das Rücksendeformular

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

It breaks down roughly like this:

• Rücksende- = return / sending back
• Formular = form

So Rücksendeformular is literally something like return form or send-back form.

German often builds long nouns this way, where English would use several separate words.

Usually, no, not if you mean fill out a form.

• ausfüllen = to fill out a form, application, questionnaire, etc.
• füllen by itself usually means to fill something physically, such as a glass, a bottle, or a bag.

So:

• Ich fülle das Rücksendeformular aus. = correct for completing the form
• Ich fülle das Glas. = I fill the glass

It is capitalized because it is the first word of the sentence.

Unlike English, German does not capitalize the pronoun ich just because it means I.
If it appears in the middle of a sentence, it is normally lowercase:

• Heute fülle ich das Rücksendeformular aus.

So Ich is capitalized here only because it starts the sentence.

Yes. German word order is flexible, but the conjugated verb still usually stays in second position in a main clause, and the separable prefix still goes to the end.

For example:

• Ich fülle das Rücksendeformular aus.
• Heute fülle ich das Rücksendeformular aus.
• Das Rücksendeformular fülle ich heute aus.

The emphasis changes, but the basic structure with fülle ... aus remains the same.

In a subordinate clause, the separable verb is usually not split. The full verb goes to the end.

Example:

• Ich fülle das Rücksendeformular aus.
• ..., weil ich das Rücksendeformular ausfülle.

That is a very important pattern with separable verbs:

• main clause: split
• subordinate clause: not split

Yes—German often writes compound nouns as one word, where English uses two or more words.

So English return form becomes German Rücksendeformular.

This is completely normal in German, and the last part of the compound usually tells you what kind of thing it is. Here, the last part is Formular, so the whole word is a kind of form.