Das Baby fängt an zu schreien, sobald der Schnuller auf den Boden fällt.

Because Baby is a neuter noun in German, so it takes the article das.

• das Baby = the baby

This is grammatical gender, not biological sex. Even if the baby is a boy or a girl, the noun Baby is still grammatically neuter.

anfangen means to begin or to start.

In a main clause, anfangen is a separable verb, so it splits:

• Das Baby fängt an ...
• literally: The baby starts ...

The conjugated part, fängt, goes in the normal verb position, and the separable prefix an goes later in the clause.

So:

• anfangen = infinitive
• fängt ... an = present tense form in a main clause

This is very common with separable verbs in German.

After anfangen, German normally uses zu + infinitive:

• anfangen zu schreien = to start crying/screaming

This works like English to start to cry, although English often also allows to start crying without a separate to before the second verb.

A useful pattern to remember is:

• anfangen zu + infinitive
• beginnen zu + infinitive

Examples:

• Ich fange an zu lesen. = I start reading.
• Sie beginnt zu lachen. = She begins to laugh.

Because when German uses zu + infinitive, the infinitive usually goes toward the end of its clause or verb phrase.

So in:

• Das Baby fängt an zu schreien

the full verbal idea is fängt an zu schreien, and schreien comes at the end of that structure.

This is normal in German and often feels more end-loaded than English.

Because sobald introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: Das Baby fängt an zu schreien
• subordinate clause: sobald der Schnuller auf den Boden fällt

That final position of fällt is one of the most important German word order rules to learn.

Other subordinating conjunctions that do the same thing include:

• weil
• dass
• wenn
• obwohl

sobald means as soon as.

It shows that one action happens immediately when another action happens.

So here the idea is:

• the pacifier falls to the floor
• immediately after that, the baby starts crying

In many situations, wenn could also be possible, but sobald is more precise because it emphasizes immediacy.

Because fallen expresses movement toward a place, so German uses the accusative after auf here.

• auf den Boden fallen = to fall onto the floor

German two-way prepositions like auf use:

• accusative for direction/movement toward somewhere
• dative for location/stillness

Compare:

• Der Schnuller fällt auf den Boden.
  The pacifier falls onto the floor.
  → movement, so accusative

• Der Schnuller liegt auf dem Boden.
  The pacifier is lying on the floor.
  → location, so dative

This is a very common contrast in German.

Because Schnuller is a masculine noun:

• der Schnuller = the pacifier/dummy

And in this sentence it is the subject of the subordinate clause:

• der Schnuller ... fällt = the pacifier falls

Subjects are in the nominative case, so der is correct here.

Yes. Both are correct.

• Das Baby fängt an zu schreien.
• Das Baby beginnt zu schreien.

Both mean The baby starts/begins to cry.

The main difference is style:

• anfangen is very common in everyday speech
• beginnen can sound a bit more neutral or slightly more formal

In normal conversation, anfangen is extremely natural.

Because German uses a comma to separate a main clause from a subordinate clause.

Here:

• Das Baby fängt an zu schreien = main clause
• sobald der Schnuller auf den Boden fällt = subordinate clause

So the comma is required.

German punctuation is often stricter than English in this area.

Literally, schreien often means to scream, to shout, or to cry loudly.

With babies, German often uses schreien where English might simply say cry.

So in this sentence, a natural English meaning is probably starts crying, even though the German verb is schreien.

If you translated it too literally as starts screaming, that could sound a bit stronger in English than the German sometimes feels.

Yes. You can start with the sobald clause:

• Sobald der Schnuller auf den Boden fällt, fängt das Baby an zu schreien.

That is also correct.

When the subordinate clause comes first, the main clause still follows the normal German rule that the finite verb comes early, so you get:

• ..., fängt das Baby ...

not:

• ..., das Baby fängt ...

This is because the entire subordinate clause takes the first position, and then the finite verb of the main clause comes immediately after it.