Bevor wir nach draußen gehen, gehe ich die Treppe hinunter und hole den Regenschirm.

Because bevor introduces a subordinate clause. In German subordinate clauses, the conjugated verb usually goes to the end.

So:

• wir gehen nach draußen = main clause
• bevor wir nach draußen gehen = subordinate clause

That is why gehen is last.

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

So the structure is:

• Bevor wir nach draußen gehen, = subordinate clause
• gehe ich die Treppe hinunter und hole den Regenschirm. = main clause

That comma is required in standard German.

In a German main clause, the conjugated verb must be in second position.

The whole clause Bevor wir nach draußen gehen takes the first position in the sentence. That means the verb of the main clause has to come next:

• Bevor wir nach draußen gehen, gehe ich ...

This is a very common pattern in German:

• Wenn ich Zeit habe, gehe ich nach Hause.
• Heute gehe ich nach Hause.

The first element can be a time expression, a subordinate clause, an object, and so on. The verb still stays second.

draußen means outside in a location sense.

• Wir sind draußen. = We are outside.

nach draußen shows movement toward the outside.

• Wir gehen nach draußen. = We are going outside.

So:

• draußen = outside, as a place
• nach draußen = out / to the outside

That is why nach is used here: the sentence is about movement.

German often expresses movement along a path with an accusative noun phrase plus a directional word.

So:

• die Treppe hinuntergehen = to go down the stairs

Here:

• die Treppe is the path being taken
• hinunter gives the direction: downward

This is a normal German pattern. Similar examples are:

• den Berg hinaufgehen = go up the mountain
• den Fluss entlanggehen = walk along the river

English uses a preposition more often; German often uses this kind of construction instead.

hinunter means downward or down in a directional sense.

In this sentence, it tells you the direction of the movement:

• gehe ich die Treppe hinunter = I go down the stairs

It is closely related to hinab, herunter, and the more colloquial runter. In everyday speech, runter is very common:

• Ich gehe die Treppe runter.

But hinunter sounds a bit more standard or written.

Traditionally:

• hinunter = movement away from the speaker’s point of view
• herunter = movement toward the speaker’s point of view

In real everyday German, many speakers do not keep this distinction very strictly, especially in casual speech. Very often both simply mean down.

In your sentence, hinunter sounds natural because the speaker is describing going downstairs, not specifically moving toward where the speaker is standing.

Because Regenschirm is the direct object of holen, and direct objects usually take the accusative case.

Regenschirm is masculine:

• nominative: der Regenschirm
• accusative: den Regenschirm

So:

• Ich hole den Regenschirm. = I’m getting the umbrella.

Yes, but more specifically holen often means go and get, fetch, or pick up something.

So hole den Regenschirm suggests:

• the umbrella is not already in the speaker’s hand
• the speaker goes to it and gets it

That makes it a very natural choice here, since the speaker first goes downstairs and then gets the umbrella.

German very often uses the present tense to talk about the near future, especially when the context already makes the timing clear.

Here, before we go outside clearly shows that this is about the next thing that will happen.

So German does not need a special future form here. The present tense is completely natural:

• Bevor wir nach draußen gehen, gehe ich die Treppe hinunter und hole den Regenschirm.

English does something similar:

• Before we go out, I get the umbrella can work in some contexts, but English usually prefers I’ll get the umbrella. German is more comfortable using the present tense this way.

Yes. The order of the verbs suggests that sequence:

• gehe ich die Treppe hinunter
• und hole den Regenschirm

So the natural reading is:

1. I go downstairs
2. I get the umbrella

Because the same subject ich does both actions, German does not need to repeat ich before hole.

Yes. German allows that too:

• Ich gehe die Treppe hinunter und hole den Regenschirm, bevor wir nach draußen gehen.

This means the same thing.

The main difference is emphasis:

• starting with Bevor wir nach draußen gehen puts the time relationship first
• putting it at the end makes the main action come first

Both are correct and natural.