Learn Hardcore German: Diese Gelegenheit nutze ich selten, denn das frühe Aufstehen ist schwierig. - I rarely use this opportunity, because getting up early is difficult.

In German main clauses the finite verb always occupies the second “slot.” Whatever you put first—in this case the object Diese Gelegenheit—pushes the subject (ich) to the third position. This fronting is purely a matter of emphasis or style; you’re making “this opportunity” the topic of your sentence.

Can I also say Ich nutze diese Gelegenheit selten? Does that change the meaning?

Yes—you can say Ich nutze diese Gelegenheit selten and the core meaning (“I seldom take advantage of this opportunity”) remains the same. The difference is that in your original sentence you’re highlighting Diese Gelegenheit by putting it first. In Ich nutze… the focus is more neutral, emphasizing the subject ich.

Why is denn used here instead of weil, and how does that affect word order?

denn and weil both mean “because,” but they behave differently:

denn is a coordinating conjunction. It does not send the verb to the end, so both clauses keep the normal S-V-O order.
weil is a subordinating conjunction. It would push its verb to the very end:
Ich nutze diese Gelegenheit selten, weil das frühe Aufstehen schwierig ist.

Using denn gives a slightly more spoken, causal tone without rearranging word order.

Why is das frühe Aufstehen capitalized and why does it take the article das?

Here Aufstehen is a nominalized infinitive—it’s a verb turned into a noun. In German, all nominalized verbs are capitalized and treated as neuter:

You always use das with a nominalized infinitive, so das Aufstehen = “the getting up.”
When you add an adjective (früh), it follows the normal article-adjective-noun pattern: das frühe Aufstehen.

Why does frühe get an -e ending here?

After a definite article (das), adjectives take the weak declension. In the nominative singular neuter, the weak ending is -e. So you get das frühe Aufstehen rather than das frühes Aufstehen.

What’s the difference between schwierig and schwer, and why choose schwierig here?

Both adjectives can mean “difficult,” but:

schwer often emphasizes heaviness or physical difficulty (“a heavy box,” “a hard blow”), or can describe abstract difficulty as well.
schwierig is more idiomatic for tasks or situations that are mentally or circumstantially challenging.

Since getting up early is seen as an abstract hardship, schwierig is the more natural choice.

What role does selten play, and can I move it elsewhere in the sentence?

selten is an adverb of frequency modifying the verb nutze. In German it commonly appears near the end of a clause, but it’s flexible:

Ich nutze diese Gelegenheit selten. (standard)
Selten nutze ich diese Gelegenheit. (emphatic fronting of the adverb)
Ich nutze selten diese Gelegenheit. (possible but less common)