Großschreibung.
If it's a noun, it's capitalized.

English capitalizes names and the first word of a sentence. German capitalizes something much bigger: word class. Every noun in German starts with a capital letter, no matter where it sits in the sentence. In fact, every single word wrapped in a colored box on this blog is a noun, a verb with fixed case government, or a pronoun, and the verbs among them stay lowercase. That is not a style choice. It is the rule, and German spelling follows it predictably.

Rule 1: nouns are always capitalized

It does not matter whether a noun opens the sentence, sits in the middle, or comes at the end. If it is a noun, in any case, it gets a capital letter.

• DerTisch ist groß. (The table is big.)
• Ich habeHunger. (I am hungry, literally: I have hunger.)
• Ich helfedemLehrer. (I help the teacher.)

Notice Hunger in the second sentence. It is an abstract idea, not a thing you can hold, and it still gets a capital letter, because it is a noun. Position in the sentence makes no difference at all.

Related: Nomen. The thing-words.

Rule 2: the first word of every sentence

This part works just like English. The first word of a sentence is always capitalized, whatever word class it belongs to. The difference shows up the moment that same word moves out of first position.

• Schnell lief er nach Hause. (Quickly he ran home.)
• Er lief schnell nach Hause. (He ran home quickly.)

Schnell is an adverb, not a noun. It only gets a capital in the first sentence because it happens to open it. Move it to the middle, and it drops straight back to lowercase. A noun, on the other hand, keeps its capital letter everywhere, as Rule 1 showed.

Rule 3: Sie, the formal you, is always capital

There is exactly one pronoun that breaks the "only nouns and sentence-openers" pattern: the formal Sie (you, formal) and its forms Ihnen and Ihr (your, formal). These are always capitalized, anywhere in the sentence, simply as a sign of respect.

• Sie sind herzlich willkommen. (You are warmly welcome, formal)
• Ich dankeIhnen sehr. (Thank you very much, formal)

Compare that with the informal du, dich, and dein. These stay lowercase, except at the start of a sentence, where Rule 2 already applies to them like any other word.

• Du bist herzlich willkommen. (You are warmly welcome, informal)

So: capital Sie always means formal "you." Lowercase du always means informal "you," capital letter or not.

Rule 4: when other words act like nouns, they get capitalized too

This is the rule that surprises learners most, and it follows directly from Rule 1. If any word, whatever its original word class, starts behaving like a noun (taking an article, filling a subject or object slot, getting a case), it gets capitalized like one. German calls this Substantivierung, its own term for nominalization.

The clearest case: an infinitive used as a noun for the activity itself.

• Schwimmen ist gesund. (Swimming is healthy.)
• Das machtdasLernen leichter. (That makes learning easier.)

Schwimmen is the infinitive of the verb schwimmen, but here it is the subject of the sentence, so it gets a capital letter and a yellow, Nominativ box, exactly like any other noun.

The same happens with adjectives, once they stand in for a noun instead of describing one.

• Ich magdasNeue. (I like the new, i.e. new things.)
• Ich habe etwas Wichtigesvergessen. (I forgot something important.)

In normal use, neu and wichtig are lowercase adjectives describing a noun → ein neues Auto, eine wichtige Frage. The moment they stand alone and act as the noun itself, after das, etwas, nichts, or viel, they get a capital letter and a case box, just like dasBuch would.