Learn Hardcore German: Übermorgen muss ich die Miete erhöhen, weil die Nebenkosten gestiegen sind. - The day after tomorrow I have to raise the rent because the utility costs have risen.

Übermorgen means the day after tomorrow. It’s a time adverb (not a noun), so normally you’d write übermorgen in lower-case when it appears mid-sentence. Here it’s capitalized simply because it’s the very first word of the sentence.

Why does the sentence start with Übermorgen muss ich instead of Ich muss übermorgen?

German follows the “verb-second” (V2) rule in main clauses: the finite verb must occupy the second position. By placing the time adverb Übermorgen first, the verb muss comes immediately after, and the subject ich moves to third position. You could still say Ich muss übermorgen …, but fronting the time expression is common for emphasis or stylistic reasons.

What type of verb is muss, and why is it followed by erhöhen without zu?

Muss is the first-person singular form of the modal verb müssen (must/have to). In German, when you use a modal verb, you conjugate it and then place the main action verb (here erhöhen) in its bare infinitive form at the end of the clause—no zu needed.

Why is the object die Miete in the accusative case, and how do we know it’s feminine?

Miete (“rent”) is a feminine noun: its nominative singular article is die. Because die Miete is the direct object of the verb erhöhen (“to raise”), it takes the accusative case. The accusative form for feminine nouns is also die, so you see die Miete erhöhen.

What exactly are Nebenkosten, and why is this word plural?

Nebenkosten literally means “additional costs” or “ancillary costs” (e.g. heating, water, trash collection). In German it’s always used in the plural, because it refers to a bundle of different bills. That’s why you have die Nebenkosten (plural) here.

Why do we say gestiegen sind instead of haben gestiegen?

Steigen (“to rise”) is an intransitive verb of change, and in German such verbs form their perfect tense with the auxiliary sein rather than haben. Hence you get sind gestiegen = “have risen.”

Why is gestiegen sind placed at the end of the clause after weil?

Weil is a subordinating conjunction, which in German pushes the finite verb to the very end of its clause. In perfect tense this means the past participle (gestiegen) is immediately before the auxiliary verb (sind), and both elements sit at the clause’s end.

Why is there a comma before weil?

In German, you must separate a main clause from a subordinate clause with a comma when you use subordinating conjunctions like weil, dass, obwohl, etc. That comma signals the start of the subordinate clause here.

Could I use denn instead of weil, and what would change?

Yes. Denn is a coordinating conjunction meaning “because,” but unlike weil, it does not send the verb to the end. If you used denn, the sentence would read:
Übermorgen muss ich die Miete erhöhen, denn die Nebenkosten sind gestiegen.
No comma rule change for coordinating conjunctions, and word order stays subject–verb.