Jeg lægger nøglen i min taske, før jeg går ud.

Because lægge means to put / to lay / to place something somewhere, while ligge means to lie / to be lying somewhere.

In this sentence, the subject is actively moving the key into the bag, so Danish uses lægger.

• Jeg lægger nøglen i min taske = I put the key in my bag.
• Nøglen ligger i min taske = The key is in my bag / The key is lying in my bag.

A useful shortcut:

• lægge = action you do to something
• ligge = where something is

Because Danish usually marks the by adding it to the end of the noun.

So:

• en nøgle = a key
• nøglen = the key

This is one of the biggest differences from English. Instead of putting a separate word like the in front, Danish often attaches definiteness to the noun itself.

Also note that nøgle becomes nøglen in the definite singular form.

Because after a possessive like min, din, hans, vores, etc., Danish normally uses the noun in its indefinite form.

So you say:

• min taske = my bag
• not min tasken

That is because the possessive already makes the noun definite enough. English does the same idea in a different way: we say my bag, not my the bag.

Because før jeg går ud is a subordinate clause, and many Danish writers put a comma before subordinate clauses.

However, this is a bit special in Danish punctuation:

• with start comma, you write the comma before the subordinate clause
• without start comma, you may leave it out

So both of these can be seen:

• Jeg lægger nøglen i min taske, før jeg går ud.
• Jeg lægger nøglen i min taske før jeg går ud.

Both can be acceptable, depending on the comma style being used.

Because før introduces a subordinate clause, and subordinate clauses do not use normal Danish main-clause word order.

In a main clause, Danish usually follows the verb-second rule:

• Jeg går ud.

But after a subordinating word like før, fordi, når, hvis, etc., the subject usually comes before the finite verb:

• før jeg går ud

So this is standard subordinate-clause word order.

Because gå ud is a verb plus a particle, similar to English go out.

So:

• går = go / am going
• ud = out

Together they form the meaning go out / leave.

This is very common in Danish:

• tage med
• gå ind
• komme hjem
• sætte sig

The little second part often changes or sharpens the meaning of the verb.

Here, i is used where English would often say into.

Danish often uses i both for:

• being inside something
• movement into something

The verb often makes the meaning clear.

So in:

• Jeg lægger nøglen i min taske

the motion comes from lægger, so it is understood as put the key into my bag, not just in my bag as a static location.

If you want extra emphasis on movement inward, Danish can also use ind i, but here plain i is completely natural.

They are both in the present tense.

• lægger = present tense of lægge
• går = present tense of gå

Even though the sentence can refer to something happening just before another future action, Danish often uses the present tense in this kind of time clause.

That is similar to English:

• before I go out not
• before I will go out

So the Danish present tense here is perfectly normal.

Because there are two clauses, and each clause needs its own subject.

The sentence has:

1. the main clause: Jeg lægger nøglen i min taske
2. the subordinate clause: før jeg går ud

Each clause has its own verb, so the subject must also be stated in each one.

English does the same:

• I put the key in my bag before I go out

You would not leave out the second I, and Danish works the same way.

Yes, in many situations you can.

Both før and inden can mean before in time.

So these are both possible:

• før jeg går ud
• inden jeg går ud

In everyday Danish, før is very natural here. Inden is also common and can sound slightly more explicitly temporal in some contexts, but there is a lot of overlap.

So for this sentence, før is completely normal.

It usually means goes out / leaves, not necessarily walks in a literal way.

Even though gå originally means walk, Danish often uses it in broader everyday ways. In gå ud, the expression often just means go out, head out, or leave the house/building, depending on context.

So you should usually understand jeg går ud idiomatically, not as strictly I walk out on foot.