An dem alten Strauch sehe ich schon eine erste Knospe.

Because an can take either the dative or the accusative, depending on the meaning.

• Dative is used for a location: where something is
• Accusative is used for direction/movement toward something

In this sentence, the bud is being seen on / at the old shrub as a location, so German uses the dative:

• an dem alten Strauch

If it were about movement toward the shrub, you might get the accusative instead.

Also, an dem is very often shortened to am in normal German:

• Am alten Strauch sehe ich schon eine erste Knospe.

That is probably the more natural everyday version.

German main clauses usually follow the verb-second rule. That means the finite verb must be in the second position.

Here, the sentence starts with a prepositional phrase:

• An dem alten Strauch = position 1

So the verb must come next:

• sehe = position 2

Then the subject comes after it:

• ich

So:

• An dem alten Strauch sehe ich schon eine erste Knospe.

If you started with the subject, you would say:

• Ich sehe an dem alten Strauch schon eine erste Knospe.

Both are grammatical, but the original sentence gives a bit more attention to the old shrub.

It is in the accusative case because it is the direct object of sehen.

You are seeing what?

• eine erste Knospe

So the structure is:

• ich = subject
• sehe = verb
• eine erste Knospe = direct object in the accusative

Because Knospe is a feminine noun, the accusative singular article is eine, which looks the same as the nominative feminine.

The adjective ending comes from the pattern used after an ein-word such as ein, eine.

Here we have:

• eine
  • erste
    • Knospe

Since Knospe is feminine singular and the phrase is accusative, the adjective takes -e:

• eine erste Knospe

This is part of German adjective declension. In this kind of phrase, the article already shows a lot of the grammatical information, so the adjective often takes a lighter ending.

Here schon means already.

It suggests that the speaker notices the bud earlier than expected, or that a process has begun:

• I can already see a first bud

It often adds a sense like:

• sooner than expected
• at this point already
• now there is already evidence of growth

So schon is not just about time; it can also add a feeling of pleasant surprise or early development.

This is a nuance question.

• eine erste Knospe means something like a first bud
• die erste Knospe means the first bud

With eine erste Knospe, the speaker is noticing one early bud, without necessarily making a strict claim that it is the single very first one in existence.

It can sound a bit like:

• one first bud
• an initial bud
• the first sign of budding

With die erste Knospe, the statement would sound more definite and specific.

So eine erste Knospe is a natural way to say that the plant is starting to bud.

Yes. Am is the normal contraction of an dem:

• an dem → am

So these are equivalent:

• An dem alten Strauch sehe ich schon eine erste Knospe.
• Am alten Strauch sehe ich schon eine erste Knospe.

In everyday German, am alten Strauch is very common and usually sounds more natural.

The full form an dem may sound a bit more deliberate or slightly more formal, or it may be used when someone wants extra emphasis.

With plants, trees, walls, and similar surfaces or edges, German often uses an to mean something like:

• on
• at
• attached to
• growing on

A bud appears on a shrub, so an is very natural here:

• an dem Strauch = on the shrub / on the bush

English and German do not always choose the same preposition, so it is best to learn this as a normal German collocation.

• der Strauch = shrub, bush → masculine
• die Knospe = bud → feminine

That is why you get:

• dem alten Strauch
  because Strauch is masculine dative singular

and

• eine erste Knospe
  because Knospe is feminine accusative singular

Usually yes, alt means old here.

So der alte Strauch is the old shrub / bush. Depending on context, that could mean:

• the shrub has been there for a long time
• it is mature
• it is not newly planted

It does not necessarily mean the shrub is dying or in bad condition; it often just means it is long-established.

Yes. German allows several word orders in the middle of the sentence, as long as the main grammar rules are respected.

For example:

• Ich sehe an dem alten Strauch schon eine erste Knospe.
• Ich sehe schon an dem alten Strauch eine erste Knospe.

These are all possible, but they place emphasis differently.

The original version:

• An dem alten Strauch sehe ich schon eine erste Knospe.

puts the spotlight first on where the speaker is looking. That makes it feel a bit more descriptive or observational.

Not only. Knospe can refer to a bud on a plant more generally, including:

• leaf buds
• flower buds
• early new growth

So in this sentence, unless the wider context tells you more, Knospe just means a bud beginning to appear on the shrub.

Because schon is an adverb, not an adjective here.

Adverbs do not change for case, gender, or number. So schon stays the same no matter what:

• Ich sehe schon ...
• Wir sehen schon ...
• Sie sahen schon ...

By contrast, adjectives used before nouns do change:

• alte
• alten
• erster
• erste, etc.