Learn Hardcore German: Der Leuchtturm zeigt nachts sowohl den Segelbooten als auch den großen Schiffen den Weg. - At night the lighthouse shows the way to both the sailboats and the big ships.

In German every noun has a grammatical gender. Leuchtturm (light-tower) is classified as masculine, so it takes the definite article der. You’ll need to learn the gender of each new noun (often given in dictionaries).

What part of speech is nachts, and how does it differ from in der Nacht?

nachts is an adverb of time meaning “at night” in a general or habitual sense. It never changes form.
By contrast in der Nacht is a prepositional phrase (“in the night”) referring to a specific night or a particular time span.

What does the correlative pair sowohl … als auch mean, and how is it used?

sowohl … als auch corresponds to English “both … and.” It links two parallel elements (nouns, clauses, etc.).
Structure:
• sowohl + first element
• als auch + second element
No comma is needed between them.

Why do den Segelbooten and den großen Schiffen both have the article den and the ending -en?

The verb zeigen here assigns these two groups as indirect objects (they receive the guidance), so they are in the dative case. In German:
• Dative plural always takes the article den.
• Most plural nouns add -n (or -en) in the dative.
Hence die Segelboote → den Segelbooten, and die großen Schiffe → den großen Schiffen.

Why is den Weg in the accusative case without a preposition?

zeigen is a ditransitive verb: it takes an indirect object (dative) and a direct object (accusative). The direct object answers “what is shown?” Here it’s der Weg, which in accusative becomes den Weg. No preposition is required because zeigen governs its objects directly.

Why isn’t there a comma before als auch?

sowohl … als auch is a pair of coordinating conjunctions. Coordinating conjunctions (and, or, but, sowohl … als auch, etc.) do not require commas. Commas appear only with subordinating conjunctions or to separate independent clauses under specific conditions.

What is the normal word order for indirect and direct objects in German, and can you switch them?

The default in German is Subject – Verb – Indirect Object (dative) – Direct Object (accusative). In our sentence without the time adverb, it would be:
Der Leuchtturm zeigt den Segelbooten den Weg.
You can swap indirect and direct objects for emphasis or stylistic reasons, but it’s less common. If you do, listeners pay special attention to whatever comes first.

Can nachts be moved to the beginning of the sentence, and how would that affect word order?

Yes. German is a V2 (verb-second) language. If you front a time adverb like nachts, it occupies position 1 and pushes the subject to position 3:
Nachts zeigt der Leuchtturm sowohl den Segelbooten als auch den großen Schiffen den Weg.