Learn Hardcore German: In einer Demokratie hat niemand die Macht allein. - In a democracy nobody has the power alone.

So In einer Demokratie literally means “in a democracy (as a state/condition)”, which is why einer (dative feminine) is used, not eine (nominative/accusative feminine).

Why does the verb come before niemand? Why hat niemand and not niemand hat?

German main clauses obey the verb-second (V2) rule: the conjugated verb must be in second position in the sentence.

In this sentence, the first position is taken by the prepositional phrase In einer Demokratie. That means the verb has to come next:

In einer Demokratie – first element
hat – conjugated verb
niemand die Macht allein – the rest of the sentence

If you start with the subject instead, you also get V2 word order:

Niemand hat in einer Demokratie die Macht allein.

Both orders are correct; the difference is just what you put in first position for emphasis or flow.

Is niemand treated as singular or plural? Why hat and not haben?

Niemand (“nobody”) is grammatically singular in German, just like in English.

So it takes a 3rd person singular verb form:

Niemand hat … (nobody has …)
Niemand ist … (nobody is …)

You never say Niemand haben or Niemand sind. Even though “nobody” refers to more than one person in meaning, grammatically it behaves like he/she/it.

Why is Demokratie capitalized?

In German, all nouns are capitalized, regardless of whether they are proper names or common nouns.

Demokratie is a common noun (like city, freedom, system in English), so it must be written with a capital D:

die Demokratie
die Macht
niemand (not capitalized, because it’s a pronoun, not a noun)

What gender do Demokratie and Macht have, and how do we see it in this sentence?

Demokratie is feminine: die Demokratie.
- In the dative singular, eine becomes einer, so we get in einer Demokratie.
Macht is also feminine: die Macht.
- Here it is in the nominative singular (as a direct object of haben), so it appears with die: die Macht.

How to know? There is no fully reliable rule, but:

Nouns ending in -ie (like Demokratie, Biologie, Theorie) are almost always feminine.
Many abstract nouns ending in -t (like Macht, Fahrt, Sicht) are feminine too. Still, you generally have to learn the gender with each noun.

Why do we say die Macht with an article? Could we also say just Macht?

In German, abstract nouns often appear with an article where English might use none.

die Macht haben = “to have (the) power”

Die Macht here refers to “the (political) power” in that democracy, understood as a specific, concrete thing, so the definite article die is natural.

You could say just Macht in some contexts (e.g. Macht haben ist verführerisch – “Having power is seductive”), but for “have the power” in the political sense, die Macht haben is the normal phrasing.

Why is allein at the end of the sentence? Could we move it?

German likes to place many adverbs and adverb-like words toward the end of the clause, especially after objects. The core phrase here is:

die Macht allein haben – “to have the power alone”

In a main clause, the conjugated verb goes to second position and any remaining verb elements or complements often go toward the end:

In einer Demokratie hat niemand die Macht allein.

You can move allein, but the nuance may change:

Niemand hat allein die Macht. – similar meaning, but slightly more emphasis on allein.
Niemand hat die Macht allein. – very natural; focus a bit more on die Macht as the thing that is not concentrated in one person.

Be careful: allein die Macht would mean “only the power”, which is different in meaning (restricting what is had, not who has it).

What is the difference between allein and alleine?

In modern standard German, allein and alleine are usually interchangeable in meaning:

both can mean “alone, by oneself”
both can also mean “only, merely”, especially allein

Some speakers perceive allein as a bit more formal or written, and alleine as more colloquial, but both are widely used.

In this sentence you could also say:

In einer Demokratie hat niemand die Macht alleine.

The meaning would be the same.

Can we replace niemand with keiner? For example: In einer Demokratie hat keiner die Macht allein.

Yes, that is possible:

In einer Demokratie hat keiner die Macht allein.

The meaning is almost the same. Differences:

niemand = “nobody, no one” (always singular, does not change form)
keiner = “no one / none”, but it declines by gender, case, and number (e.g. keiner, keine, kein).

Niemand sounds a bit more neutral/formal, while keiner can sound slightly more colloquial or emphatic, depending on context. In this sentence, both are fine.

What is the difference between In einer Demokratie and In der Demokratie?

In einer Demokratie = “in a democracy” in general, as a type of political system. It is generic, talking about democracies as a concept.
In der Demokratie = “in the democracy”, referring to one specific democracy that is known from context (for example, the democracy of a particular country you have been discussing).

So in a general statement about how democracies work, In einer Demokratie is the more natural choice.

Could we change the word order to Niemand hat in einer Demokratie die Macht allein? Is that still correct?

Yes, that sentence is perfectly correct:

Niemand hat in einer Demokratie die Macht allein.

Now the subject Niemand is in first position, so the verb hat still occupies the mandatory second position. The prepositional phrase in einer Demokratie has just moved to later in the sentence.

Both versions are fine:

In einer Demokratie hat niemand die Macht allein. – starting with the setting/context.
Niemand hat in einer Demokratie die Macht allein. – starting with “nobody” for stronger focus on the subject.

The choice is mostly about emphasis and style, not grammar.