Mein Fahrradschloss, das ich sonst im Rucksack trage, liegt noch zu Hause.

Because Fahrradschloss is a neuter noun: das Fahrradschloss.

In the nominative singular, the possessive word changes like this:

• masculine: mein
• neuter: mein
• feminine: meine
• plural: meine

So:

• mein Hund
• mein Fahrradschloss
• meine Tasche
• meine Bücher

Here, Fahrradschloss is the subject of the sentence, so nominative is used: Mein Fahrradschloss ... liegt ...

It is a very typical German compound noun:

• Fahrrad = bicycle
• Schloss = lock

Together: Fahrradschloss = bike lock / bicycle lock

In German, compound nouns are usually written as one word. The last part determines the gender, so because das Schloss is neuter, das Fahrradschloss is also neuter.

This is a relative clause. It gives extra information about Mein Fahrradschloss.

So the sentence structure is:

• Mein Fahrradschloss = the thing being talked about
• das ich sonst im Rucksack trage = extra information about it
• liegt noch zu Hause = the main statement

In English, this is like:

• My bike lock, which I usually carry in my backpack, is still at home.

The commas mark off the relative clause.

Because here das is a relative pronoun, not the conjunction dass.

In this sentence, das refers back to Fahrradschloss:

• das Fahrradschloss
• das ich sonst im Rucksack trage

So das means which/that.

By contrast, dass means that in the sense of introducing a content clause:

• Ich weiß, dass du müde bist. = I know that you are tired.

A useful test:

• If it refers back to a noun, it is often das
• If it introduces a statement or idea, it is often dass

Because it must match the noun it refers to in gender and number, but its case depends on its function inside the relative clause.

It refers back to das Fahrradschloss, which is:

• singular
• neuter

So the possible forms would be:

• nominative: das
• accusative: das
• dative: dem
• genitive: dessen

Inside the relative clause, the lock is the direct object of trage:

• ich trage das = I carry it

That means the relative pronoun is accusative, and for neuter singular accusative, the form is also das.

Because German relative clauses usually send the conjugated verb to the end.

Main clause:

• Mein Fahrradschloss liegt noch zu Hause.

Relative clause:

• das ich sonst im Rucksack trage

This is the normal pattern for subordinate clauses in German:

• ..., weil ich es im Rucksack trage.
• ..., das ich im Rucksack trage.

So even though trage is the finite verb, it goes to the end because the clause is subordinate.

Here sonst means something like:

• normally
• usually
• otherwise / in other circumstances

In this sentence, sonst tells us what the usual situation is:

• das ich sonst im Rucksack trage
• which I normally carry in my backpack

So the idea is: I usually carry it in my backpack, but this time it is still at home.

im is a contraction of in dem.

• in dem Rucksack → im Rucksack

This is extremely common in German.

Other common contractions:

• am = an dem
• vom = von dem
• zum = zu dem
• beim = bei dem

Here, im Rucksack means in the backpack.

German often uses verbs like liegen, stehen, and hängen to describe where something is.

• liegen = to lie, to be lying
• stehen = to stand
• hängen = to hang

A lock is thought of as something lying somewhere, so:

• Das Fahrradschloss liegt zu Hause.

This sounds more natural in German than just ist zu Hause.

Very roughly:

• Das Buch liegt auf dem Tisch.
• Die Flasche steht auf dem Tisch.
• Das Bild hängt an der Wand.

So liegt gives a more physical sense of where the object is.

Here noch means still.

• liegt noch zu Hause = is still at home

It suggests that the lock has not yet been brought along. The speaker probably expected to have it with them, but it remains at home.

Common meanings of noch include:

• still
• yet
• another / one more, depending on context

In this sentence, still is the right idea.

Yes. Both forms exist, but they are used a little differently.

In your sentence, zu Hause is a very standard way to say at home:

• Ich bin zu Hause.
• Mein Fahrradschloss liegt noch zu Hause.

You may also see zuhause, especially as an adverb:

• Ich bin zuhause.

Both are common, but many learners first meet zu Hause as the classic form.

Also note:

• zu Hause = at home
• nach Hause = homeward / to home

For example:

• Ich bin zu Hause. = I am at home.
• Ich gehe nach Hause. = I am going home.

Not really. The main clause is still:

• Mein Fahrradschloss liegt noch zu Hause.

The relative clause is just inserted after the subject:

• Mein Fahrradschloss, das ich sonst im Rucksack trage, liegt noch zu Hause.

So the main verb liegt is still the verb of the main clause. It may look far away from the subject because the relative clause interrupts the sentence, but structurally it still belongs to the main clause.

A useful way to see it is to mentally remove the relative clause:

• Mein Fahrradschloss liegt noch zu Hause.

That often makes German sentences much easier to understand.