Learn Hardcore German: Die Tasche ist leer, aber eine alte Quittung liegt noch darin. - The bag is empty, but an old receipt is still in it.

In this sentence, die Tasche is the subject of the first clause (Die Tasche ist leer), so nominative is required, and the nominative feminine article is die.

There is no general rule that tells you Tasche must be feminine; it’s something you usually learn with the noun:
die Tasche – bag, pocket, handbag.

What exactly does leer mean here, and why doesn’t it have an ending (leere)?

leer means empty.

In Die Tasche ist leer, leer is a predicate adjective after the verb sein (ist). Predicate adjectives in German do not take adjective endings:

Die Tasche ist leer. – The bag is empty.
Das Glas ist voll. – The glass is full.
Das Auto ist neu. – The car is new.

You only add endings when the adjective directly comes before a noun:

die leere Tasche – the empty bag
ein leeres Glas – an empty glass

Why is it aber here? Could I also use doch, jedoch or sondern?

In this context, aber is the normal translation of “but”:

Die Tasche ist leer, aber eine alte Quittung liegt noch darin.
The bag is empty, but an old receipt is still lying inside it.

The contrast is: empty vs. still containing something.

Other options:

doch – also means something like but / however, often with a slightly more emotional, contradicting tone.
Die Tasche ist leer, doch eine alte Quittung liegt noch darin. (a bit more literary / formal)
jedoch – more formal, like however.
Die Tasche ist leer, jedoch liegt noch eine alte Quittung darin.
sondern – only after a negation, like but rather:
You’d use it in a sentence like:
Die Tasche ist nicht leer, sondern es liegt noch eine alte Quittung darin.
(“The bag is not empty, but rather there is still an old receipt in it.”)

So aber is the most neutral and idiomatic choice in the given sentence.

Why is it eine alte Quittung and not ein alter Quittung?

Three things are happening here:

Quittung is feminine
→ its indefinite article in nominative singular is eine, not ein.
eine alte Quittung is the subject of the second clause
→ nominative case is used, and feminine nominative singular takes the ending -e on the adjective.
Adjective endings after eine (feminine, nominative) are -e.

So:

Article: eine (feminine nominative)
Adjective: alte (feminine nominative ending -e)
Noun: Quittung

Compare:

eine alte Tasche (fem.)
ein alter Tisch (masc.)
ein altes Auto (neut.)

What does Quittung mean exactly, and how is it different from Rechnung?

Quittung usually means a receipt, especially something that confirms you have paid.

Quittung – receipt / written proof that money has been received.
Rechnung – bill, invoice or statement of what you owe.

Examples:

Können Sie mir bitte eine Quittung geben?
Can you please give me a receipt?
Ich habe die Rechnung noch nicht bezahlt.
I haven’t paid the bill yet.

So in this sentence, eine alte Quittung is naturally understood as an old receipt that was once given for payment and is now just left in the bag.

Why does the sentence use liegt instead of ist, like „…aber eine alte Quittung ist noch darin“?

German often prefers specific “position verbs” to describe where something is:

liegen – to lie (flat or resting somewhere)
stehen – to stand (upright)
sitzen – to sit (be seated)

liegt here describes the position of the receipt in the bag, similar to:

Das Buch liegt auf dem Tisch.
The book is (lying) on the table.

Using ist is not wrong in all contexts, but it sounds more neutral/abstract. In everyday German, if you’re talking about a physical object being somewhere, you commonly use liegen / stehen / sitzen / stecken, etc.

So:

Eine alte Quittung liegt noch darin.
An old receipt is still lying inside it. (natural, idiomatic)

What does noch mean here, and why is it in front of darin?

In this sentence, noch means still:

Eine alte Quittung liegt noch darin.
An old receipt is still lying inside it.

Here, noch modifies the whole “lying in there” situation: despite the bag being “empty”, one thing still remains.

Word order:

Eine alte Quittung liegt noch darin. – neutral: still
Noch liegt eine alte Quittung darin. – more emphasis on still, a bit more literary or dramatic.

Important:
If you put noch directly before eine:

Noch eine alte Quittung liegt darin.
This would usually be understood as “another old receipt is lying in there”, not “still”.

So, the position liegt noch darin clearly gives noch the meaning still.

What is darin, and how is it different from drin, da drin, or in ihr / in der Tasche?

darin literally means “in it / inside it” and refers back to die Tasche (the bag).

It’s a combination of da- (there) + in (in):

da
- in → darin

Alternatives:

drin – very common in spoken German, a shortened form:
Eine alte Quittung liegt noch drin. (colloquial, normal in speech)
da drin / da drinnen – more emphatic or explicit:
Eine alte Quittung liegt noch da drin.
in ihr / in der Tasche – more literal, using a pronoun or the full noun:
Eine alte Quittung liegt noch in ihr. (refers to the bag, feminine)
Eine alte Quittung liegt noch in der Tasche.

All can be correct, but:

darin is a bit more neutral/written style.
drin is very common in speech.
in der Tasche repeats the noun instead of referring back with a pronoun-like word.

Why are die Tasche and eine alte Quittung both in the nominative case?

Each clause has its own subject:

Die Tasche ist leer, …
– Die Tasche is the subject of the first clause.
… aber eine alte Quittung liegt noch darin.
– eine alte Quittung is the subject of the second clause.

In German, the subject of a clause takes the nominative case, so both are nominative:

die Tasche (fem. nominative singular)
eine alte Quittung (fem. nominative singular)

There is no direct object in either clause; the verbs sein (ist) and liegen do not take accusative objects here.

How does the verb-second rule work in the second part: aber eine alte Quittung liegt noch darin?

In main clauses, German generally follows the V2 rule: the finite verb is in second position.

In the second clause:

aber – coordinating conjunction (does not count as a position in V2 counting)
eine alte Quittung – first real element (subject)
liegt – second element (verb)
noch darin – rest of the clause (adverb + adverbial pronoun)

So structurally:

[aber] [Subjekt: eine alte Quittung] [Verb: liegt] [Rest: noch darin]

Compare:

Heute liegt eine alte Quittung noch darin.
(Today, an old receipt is still in there.)
Here: Heute = position 1, liegt = position 2, eine alte Quittung and noch darin come after.

Can Tasche mean both “bag” and “pocket” in German?

Yes. die Tasche can mean:

A bag / handbag / tote bag:
- Ich habe meine Tasche im Auto gelassen.
  I left my bag in the car.
A pocket (in clothing):
- In meiner Jackentasche ist noch ein Schlüssel.
  There is still a key in my jacket pocket.

Often the context tells you whether it’s a bag you carry or a pocket in clothes. In the given sentence, die Tasche is most naturally understood as a bag or handbag, but pocket is also possible depending on context.