Learn Hardcore German: Ich lege die Lederkette auf den Tisch, bevor ich ins Bett gehe. - I put the leather necklace on the table before I go to bed.

The gender of a compound noun in German is determined by the last part of the compound.
Since Kette is feminine, Lederkette is also feminine and takes die in the nominative and accusative singular.

Why is Lederkette written as one word instead of Leder Kette like “leather necklace” in English?

German normally writes compound nouns as one word:

Leder + Kette → Lederkette
Haus + Tür → Haustür (house door, front door)
Sommer + Ferien → Sommerferien (summer holidays)

So Leder here is not an adjective; it’s the first part of a compound noun.
You cannot write Leder Kette as two separate nouns in standard German.

What case is die Lederkette in, and how can I tell?

In the sentence:

Ich lege die Lederkette auf den Tisch …

ich is the subject → nominative
lege is the verb
die Lederkette is what is being laid/put → direct object → accusative

For feminine nouns, nominative and accusative singular look the same:

Nominative: die Lederkette (subject)
Accusative: die Lederkette (direct object)

So die Lederkette here is accusative, but the article doesn’t change form from the nominative. You know it’s accusative because it’s the thing being acted on by the verb.

Why is it auf den Tisch and not auf dem Tisch?

Auf is a two-way preposition. It can take:

dative (wo? = where? – location, no movement)
accusative (wohin? = to where? – movement to a destination)

Compare:

Auf dem Tisch liegt die Lederkette.
→ The necklace is (lying) on the table. (location → dative: dem)
Ich lege die Lederkette auf den Tisch.
→ I put the necklace onto the table. (movement to the table → accusative: den)

Here we have movement toward the table, so auf takes the accusative, and der Tisch (masculine) becomes den Tisch.

Why do we say den Tisch and not der Tisch?

Der Tisch is masculine. In the definite article system:

Nominative masculine: der Tisch (subject)
Accusative masculine: den Tisch (direct object / after certain prepositions with direction)

Since auf here is used with motion toward the table, it takes the accusative, so you must say auf den Tisch, not auf der Tisch or auf dem Tisch (the last would be dative, used for pure location).

What exactly is ins Bett, and why not just in das Bett or im Bett?

ins is a common contraction:

ins = in das

So:

ich gehe in das Bett
→ grammatically correct but sounds bookish/formal
ich gehe ins Bett
→ normal, idiomatic everyday German

Again, in is a two-way preposition:

wohin? (to where?) → accusative:
Ich gehe ins Bett. (movement → in + das Bett → ins Bett)
wo? (where?) → dative:
Ich liege im Bett. (location → in + dem Bett → im Bett)

So here, ins Bett = motion into the bed → accusative.

Why is the verb gehe at the very end of bevor ich ins Bett gehe?

Bevor is a subordinating conjunction. In German, subordinate clauses introduced by words like weil, dass, wenn, bevor, während send the conjugated verb to the end of the clause.

Pattern:

bevor
- subject + (rest of clause) + verb (conjugated)

So:

bevor ich ins Bett gehe
literally: before I into bed go

That’s why gehe appears at the end. You cannot say bevor ich gehe ins Bett in standard German.

Can I put the bevor-clause at the beginning of the sentence? How does the word order change?

Yes, you can. Then the entire subordinate clause counts as position 1 in the main clause, and in the main clause the conjugated verb must still be in position 2:

Original:
Ich lege die Lederkette auf den Tisch, bevor ich ins Bett gehe.
With the time clause first:
Bevor ich ins Bett gehe, lege ich die Lederkette auf den Tisch.

Notice:

In the subordinate clause: bevor ich ins Bett gehe (verb gehe at the end)
In the main clause after the comma: lege ich … (verb lege in second position, followed by the subject ich)

Why is it ins Bett gehe and not gehe ins Bett inside that clause?

Inside a subordinate clause introduced by bevor, the finite (conjugated) verb must go to the end:

Main clause:
Ich gehe ins Bett. → verb second: gehe in position 2
Subordinate clause with bevor:
bevor ich ins Bett gehe → same elements, but the verb gehe is moved to the end

So the word order change is purely because of the type of clause, not because of the verb gehen itself.

Could I say something else instead of lege here, like stelle or tue?

You have several options, but they are not all equivalent:

legen – to lay/put something down in a lying (flat) position
- works well for a necklace on a table → Ich lege die Lederkette auf den Tisch.
stellen – to put/place something upright / standing
- used for bottles, vases, etc.
- A necklace doesn’t really “stand”, so stellen sounds wrong here.
tun – very colloquial, like “to put” or even “to do”
- You might hear: Ich tue die Lederkette auf den Tisch.
- Grammatically possible but colloquial; many teachers discourage overusing tun.

So in standard, idiomatic German, legen is the best choice for a necklace placed flat on a table.

How would this sentence look in the (spoken) past tense?

The most common spoken past in German is the Perfekt (present perfect). Your sentence would become:

Ich habe die Lederkette auf den Tisch gelegt, bevor ich ins Bett gegangen bin.

Changes:

Main clause:
- lege → habe gelegt (auxiliary haben
  - past participle gelegt)
Subordinate clause with bevor:
- gehe → gegangen bin (auxiliary sein
  - past participle gegangen, both at the end of the clause)

Note the word order in the subordinate clause: the past participle (gegangen) comes before the auxiliary (bin) at the very end.