Bevor sie losfährt, stellt sie den Rückspiegel ein und legt beide Hände ans Lenkrad.

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

So:

• Bevor sie losfährt = Before she drives off

With a separable verb like losfahren, something extra happens:

• In a main clause: Sie fährt los.
• In a subordinate clause: ..., bevor sie losfährt.

So the prefix los- does not split off in the subordinate clause.

Losfahren means to set off, to drive off, or to start driving.

It is a separable verb:

• infinitive: losfahren
• main clause: Sie fährt los.
• subordinate clause: ..., bevor sie losfährt.

The prefix los- often gives the idea of beginning movement.

Because the sentence begins with the subordinate clause:

• Bevor sie losfährt, ...

When a subordinate clause comes first in German, the main clause follows with verb first, then subject. This is often called inversion.

So:

• Bevor sie losfährt, stellt sie den Rückspiegel ein.

Not:

• Bevor sie losfährt, sie stellt ... ❌

A useful pattern is:

• [subordinate clause], [verb] [subject] ...

Because einstellen is another separable verb.

Its basic pattern in a main clause is:

• Sie stellt den Rückspiegel ein.

So:

• stellt = the conjugated verb
• ein = the separable prefix, which goes to the end of the clause

Here einstellen means to adjust.

Compare:

• Sie stellt den Rückspiegel ein. = She adjusts the rear-view mirror.
• ..., weil sie den Rückspiegel einstellt. = ... because she adjusts the rear-view mirror.

In the subordinate clause version, the verb stays together at the end.

Because Rückspiegel is the direct object of einstellen, so it takes the accusative case.

The noun is masculine:

• nominative: der Rückspiegel
• accusative: den Rückspiegel

So:

• Sie stellt den Rückspiegel ein.

This is a very common pattern:

• der → den for masculine nouns in the accusative

Literally, Rückspiegel is something like rear mirror:

• Rück- = rear / back
• Spiegel = mirror

In this sentence it usually means rear-view mirror.

Because beide Hände is in the accusative plural, and beide is being used without an article.

Here Hände is the direct object of legt:

• Sie legt beide Hände ans Lenkrad.

With no article, beide takes the plural ending -e here:

• nominative/accusative plural: beide Hände

beiden would be used in other case patterns, for example:

• mit beiden Händen = with both hands

So:

• beide Hände = accusative plural
• beiden Händen = dative plural

Ans is just the standard contraction of an das.

So:

• ans Lenkrad = an das Lenkrad

German often contracts:

• an dem → am
• an das → ans
• in dem → im
• in das → ins

Using the contraction here is completely normal and natural.

This is about the two-way preposition an.

With two-way prepositions, German uses:

• accusative for direction / movement toward something
• dative for location / being at something

Here the hands are being placed onto / toward the steering wheel:

• Sie legt beide Hände ans Lenkrad.

That implies movement, so German uses accusative:

• an das Lenkrad → ans Lenkrad

If you were just describing location, you would usually use dative:

• Sie hat beide Hände am Lenkrad. = She has both hands on the steering wheel.

So the contrast is:

• ans Lenkrad legen = put onto the steering wheel
• am Lenkrad sein = be on the steering wheel

It simply happens to be a neuter noun in German:

• das Lenkrad

You generally have to learn the gender with the noun. Here:

• nominative: das Lenkrad
• accusative: das Lenkrad
• dative: dem Lenkrad

Since the sentence has ans = an das, you are seeing the accusative form of the neuter article.

Yes, literally it means something like puts both hands onto the steering wheel.

In natural English, depending on context, you might translate it as:

• puts both hands on the steering wheel
• places both hands on the steering wheel

German often uses a verb of placing like legen in cases where English may simply say put or place.

It could say that, but it is not necessary.

• beide Hände = both hands
• ihre beiden Hände = her two/both hands

Since it is already obvious whose hands they are from sie, German often leaves out the possessive pronoun.

This is very common in German, especially with body parts or things closely connected to the subject.

Because German often uses the present tense to describe:

• habitual actions
• instructions
• general scenes
• action sequences in a vivid, immediate way

So this sentence can naturally mean something like:

• Before she drives off, she adjusts the rear-view mirror and places both hands on the steering wheel.

Even though the actions happen in order, the present tense is perfectly normal.

Yes, but only if the clause structure changes.

In a main clause:

• Sie fährt los.

But after bevor, you need a subordinate clause, so you must say:

• bevor sie losfährt

Not:

• bevor sie fährt los ❌

So both forms belong to the same verb, but they appear in different sentence structures.

The sentence is:

• Bevor sie losfährt, stellt sie den Rückspiegel ein und legt beide Hände ans Lenkrad.

Its structure is:

1. Subordinate clause first
   
   • Bevor sie losfährt
2. Main clause with inversion
   
   • stellt sie den Rückspiegel ein
3. Second coordinated verb phrase
   
   • und legt beide Hände ans Lenkrad

So a rough pattern is:

• Before she sets off, adjusts she the rear-view mirror and places both hands on the steering wheel

Of course, that is not good English word order, but it helps show how German is built.