Adverbs never modify or describe nouns. Adverbs are used to modify verbs (hence the name adverb), but they can also be used to modify adjectives (sehr gut - very good) or another adverb (sehr gern - very gladly). In English, we can easily make an adverb out of an adjective by adding the suffix "-ly".
In German, we do not have an equivalent of "-ly" adverbs. However, a word can be used as an adjective in one sentence, and an adverb in another. For example, we can use the word "nett" (nice) as either an adverb or an adjective. Example:Der Mann ist nett. The man is nice. (adjective). Der Mann lächelt nett. The man is smiling nicely. (adverb) Notice that the adverb "nett" did not change. Adverbs in German are classified as "unveränderlich" (unchangeable), because they do not change regardless of subject or verb. (Note: a handful of adverbs can be used in comparison, which does add comparative or superlative endings). In general, adverbs help us describe the circumstances under which a verb is done. We can describe a circumstance based on location or place (Lokaladverbien), time or frequency (Temporaladverbien), manner (Modaladverbien), or cause (Kausaladverbien). This is something that they have in common with prepositions, which adds to the confusion. But remember, an adverb describes how a verb is done. Examples:
The car is beautiful. (adjective)
The car drives beautifully. (adverb)
Der Hund spielt draussen. The dog plays outside (Lokaladverb)
Der Hund spielt oft. The dog plays often. (Temporaladverb)
Der Hund spielt deshalb. The dog plays for that reason. (Kausaladverb)Der Hund spielt komisch. The dog plays weirdly. (Modaladverb).
Prepositions are words that are used to describe the relationship between one noun or pronoun (person, object, thing, etc.) to another noun or pronoun (person, object, thing, etc.) in a sentence. Because they can describe this relationship based on time (temporale Präpositionen), place (lokale Präpositionen) and manner (modale Päpositionen), there can be confusion between them and adverbs, which can describe verbs based on time, place and manner. Prepositions are also classified as "unveränderlich" (unchangeable), because the prepositions themselves do not change, regardless of the gender or case. However, as they describe the relationship between nouns (which is exactly what the cases help us demonstrate), they trigger specific cases depending on the nature of this relationship.
Der Junge steht vor dem Haus. (lokale Präposition).
The boy stands in front of the house.
Der Junge lernt vor der Prüfung. (temporale Präposition)
The boy studies before the exam.
Der Junge schreibt mit dem Bleistift. (modale Präposition)
The boy writes with the pencil.
In the above examples, the prepositions used help us demonstrate the relationship between the boy and the house, the exam, and the pencil. Some prepositions always trigger the same case (Akkusativ Präpositionen, Dativ Präpositionen, Genitiv Präpositionen), and some can flip between accusative and dative, depending on the question that is asked (Dative: Wo? -Where?; Accusative: Wohin? - Where to?, Woher? - Where from?).
What can add to the confusion between adverbs and prepositions is that they can appear side by side in a sentence.
Der Hund spielt draussen im Garten. The dog plays outside in the yard.
In this example, the adverb "draussen" (outside) describes where the boy is playing, but the prepositional phrase "im Garten" (in the yard), also describes where the boy is playing. But if we break down the sentence a bit, we can figure out that "draussen" cannot be used directly with a noun, or to describe the relationship to a noun.
For example, we could not say "Der Hund spielt draussen Garten.". That is because it is an adverb. However, the preposition "in" that is used here, can (and is) used to describe the relationship between the dog and the yard.
In brief, adjectives describe nouns. If placed before a noun in German, adjectives take endings depending on gender, case and article. Adverbs modify verbs, other adverbs or adjectives. They do not change. Both adjectives and adverbs can be used for comparison. Prepositions demonstrate the relationship between nouns and trigger certain cases.