Bevor wir losgehen, fülle ich das Fläschchen mit Wasser und lege es in die Wickeltasche.

Because bevor is a subordinating conjunction, like before in English. In a German subordinate clause, the conjugated verb normally goes to the end.

• wir gehen los = we set off / we leave
• bevor wir losgehen = before we set off

Notice that losgehen is a separable verb. In a normal main clause, it splits:

• Wir gehen los.

But in a subordinate clause, it stays together at the end:

• ..., bevor wir losgehen.

Losgehen here means something like to set off, to head out, or to leave.

It is often used when talking about starting to go somewhere:

• Wir gehen jetzt los. = We’re leaving now.
• Wann gehen wir los? = When are we setting off?

So bevor wir losgehen is more natural than just bevor wir gehen if the idea is before we head out / before we leave.

Because the sentence begins with the subordinate clause Bevor wir losgehen. In German main clauses, the finite verb must come in the second position. When something other than the subject comes first, the verb still stays second, and the subject moves after it.

So:

• Ich fülle das Fläschchen ... = normal word order
• Bevor wir losgehen, fülle ich das Fläschchen ... = the time clause comes first, so fülle comes before ich

This is often called inversion, though it is just normal German word order.

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

So in:

• Bevor wir losgehen, fülle ich ...

the comma marks the end of the bevor clause and the start of the main clause.

This comma is required in standard German.

Fläschchen is a diminutive form of Flasche.

• die Flasche = bottle
• das Fläschchen = little bottle / small bottle

The ending -chen often makes a noun sound smaller, cuter, or more specific. A very important grammar point: nouns ending in -chen are always neuter.

That is why it is:

• das Fläschchen

not die Fläschchen.

In context, this probably suggests a baby bottle or a small feeding bottle.

Because es is the pronoun that refers back to the neuter noun das Fläschchen.

German pronouns must match the grammatical gender of the noun they replace:

• masculine noun → er
• feminine noun → sie
• neuter noun → es

So:

• das Fläschchen → es

That is why the sentence says:

• ... und lege es in die Wickeltasche.

In German, mass nouns like Wasser, Milch, Kaffee, and Brot often appear without an article when you mean the substance in a general sense.

So:

• mit Wasser = with water
• mit Milch = with milk

If you said mit dem Wasser, it would usually mean with the water in a more specific sense, referring to particular water already known from context.

Here, the general meaning is enough: the bottle is being filled with water.

Because in can take either the accusative or the dative, depending on whether there is movement or location.

• accusative = movement into somewhere
• dative = location in somewhere

Here, the bottle is being placed into the diaper bag, so there is movement:

• ich lege es in die Wickeltasche = I put it into the diaper bag

Compare:

• Es liegt in der Wickeltasche. = It is in the diaper bag.

So:

• in die Wickeltasche = direction/movement
• in der Wickeltasche = location

German often leaves out the subject in the second part of a coordinated clause when it is the same as before.

So this is perfectly normal:

• ..., fülle ich das Fläschchen mit Wasser und lege es in die Wickeltasche.

The subject ich applies to both verbs:

• fülle
• lege

You could repeat ich, but it would usually sound unnecessary:

• ..., fülle ich das Fläschchen mit Wasser und ich lege es in die Wickeltasche.

That is grammatical, but less natural here.

Die Wickeltasche means diaper bag or changing bag.

It is a compound noun:

• wickeln = to change a baby / to wrap
• die Tasche = bag

So a Wickeltasche is a bag for baby-changing things.

Like most German compound nouns, the gender comes from the final part of the word:

• die Tasche → therefore die Wickeltasche

German often uses the present tense to talk about the near future, just like English does.

• Bevor wir losgehen, fülle ich ...
  = Before we leave, I’ll fill ...

This is very natural in German. You do not need a special future form here.

German Futur I is possible in some contexts, but for everyday planned actions, the present tense is much more common.

Yes, slightly.

• füllen = to fill
• einfüllen = to pour in / fill in, emphasizing putting something into a container

In this sentence, fülle ich das Fläschchen mit Wasser is completely natural and idiomatic: I fill the bottle with water.

You could also hear something like:

• Ich fülle Wasser in das Fläschchen.

That shifts the focus slightly toward the water being put in. But the original version is very standard.

Yes. In das Fläschchen mit Wasser füllen, the pattern is:

• something
  • mit
    • what it is filled with

Examples:

• ein Glas mit Saft füllen = to fill a glass with juice
• die Badewanne mit Wasser füllen = to fill the bathtub with water

So mit Wasser tells you the contents used to fill the little bottle.