Wenn meine Nichte kommt, will sie zuerst im Beet nachsehen, ob schon neue Knospen da sind.

Because wenn introduces a subordinate clause. In German, the conjugated verb normally goes to the end of a subordinate clause.

• Wenn meine Nichte kommt = When my niece comes
• Structure: wenn + subject + ... + verb

This is one of the most common word-order patterns in German.

Because the sentence begins with a subordinate clause: Wenn meine Nichte kommt. When a subordinate clause comes first, it counts as position 1 in the main clause. That means the conjugated verb of the main clause must come immediately after it, in position 2.

So:

• Wenn meine Nichte kommt, will sie ...

not

• Wenn meine Nichte kommt, sie will ...

This is a standard German verb-second pattern in main clauses.

No. German will is from wollen, which means to want or to intend to.

So:

• sie will nachsehen = she wants to check / she intends to check

It does not mean the English future marker will. This is a very common false friend for English speakers.

Nachsehen is a separable verb: nach|sehen.

In a normal main clause, it often splits:

• Sie sieht im Beet nach.

But with a modal verb like will, the second verb stays in the infinitive at the end:

• Sie will im Beet nachsehen.

So here it stays together because it is part of a verb cluster with will.

Here nachsehen means to check, to look and see, or to have a look.

In this sentence:

• im Beet nachsehen = check in the flowerbed / garden bed

It suggests looking to see whether something is there.

Because im Beet means in the flowerbed as a location. The action is happening there.

• im Beet = in the bed / in the flowerbed → location
• ins Beet = into the flowerbed → movement into it

Since she is checking there, not moving into it, German uses the dative:

• in dem Beet → im Beet

Beet usually means a flowerbed, garden bed, or plant bed.

It does not mean bed in the sleeping sense. That would be das Bett.

So:

• das Beet = garden bed / flowerbed
• das Bett = bed

Those two words are easy to confuse.

Ob means whether / if in the sense of uncertainty or indirect yes/no questions.

The clause

• ob schon neue Knospen da sind

means

• whether there are already new buds there

German uses ob when someone is checking, asking, or wondering whether something is true.

Compare:

• Ich weiß nicht, ob er kommt. = I don’t know whether he’s coming.

Because ob also introduces a subordinate clause, and subordinate clauses put the conjugated verb at the end.

So:

• ob ... da sind

not

• ob ... sind da

This is the same basic rule as with wenn.

Here schon means already.

• ob schon neue Knospen da sind = whether there are already new buds there

It suggests that the speaker expects the buds may have appeared by now.

Be careful: schon can also mean other things in different contexts, such as indeed, all right, or be part of idiomatic expressions. But here it simply means already.

Da sein is a common expression meaning to be there, to be present, or to exist/has appeared in context.

So:

• ob schon neue Knospen da sind

literally is something like:

• whether already new buds are there

Natural English:

• whether there are already new buds

The da adds the idea of presence: the buds are there / present.

Neue Knospen is nominative plural because it is the subject of the clause ob schon neue Knospen da sind.

• singular: die neue Knospe
• plural: die neuen Knospen

But without an article, the adjective takes the ending -e in the nominative/accusative plural:

• neue Knospen

So the form is correct for new buds.

German uses commas to separate subordinate clauses from the main clause.

This sentence has two subordinate clauses:

1. Wenn meine Nichte kommt
2. ob schon neue Knospen da sind

So the commas help show the structure:

• Wenn meine Nichte kommt,
• ..., ob schon neue Knospen da sind.

German comma usage is stricter than English in cases like this.

Usually no, not in this context.

• wenn = when / whenever, often for repeated events or general future situations
• als = when, but only for a single completed event in the past

So:

• Wenn meine Nichte kommt, ... = When my niece comes, ... / Whenever my niece comes, ...

If you were talking about one specific past occasion, then als would be possible:

• Als meine Nichte kam, ... = When my niece came, ...

It can move, but its current position is very natural.

• will sie zuerst im Beet nachsehen

Here zuerst clearly modifies nachsehen and means first.

Other placements are possible depending on emphasis, for example:

• ... will sie im Beet zuerst nachsehen ...

But that may sound slightly different in focus. The given version is the most neutral and idiomatic for many contexts.