Learn Hardcore German: Gestern habe ich ihn schimpfen hören, als ein Karton vor seiner Tür stand. - Yesterday I heard him complain when a box was standing in front of his door.

Because in German, when something other than the subject is put in the first position (here: Gestern), the finite verb must come next. This is the V2 rule (verb-second) in main clauses:

Gestern (position 1)
habe (finite verb, position 2)
ich (subject, after the verb)

So Gestern habe ich ... is the normal, correct word order.

Why do we use habe here—what tense is this?

habe is the auxiliary of the Perfekt (present perfect), very common in spoken German for past events:

Gestern habe ich ihn schimpfen hören = Yesterday I heard him scolding/complaining.

Even though it resembles English present perfect, in everyday German it often simply functions as the normal “past” tense in narration.

Why are there two verbs at the end: schimpfen hören?

This is a common pattern where a verb of perception (hören, sehen, fühlen, etc.) combines with another verb in the bare infinitive:

jemanden + Verb (inf.) + hören/sehen Examples:
Ich habe ihn kommen sehen.
Ich habe sie lachen hören.

Here, schimpfen describes what you perceived him doing, and hören is the perception verb.

Why is it ihn schimpfen hören and not something like ihn schimpfend hören?

German typically uses the infinitive construction after perception verbs, not an -ing form. So you say:

Ich habe ihn schimpfen hören. (standard) Using schimpfend is possible in some contexts, but it would sound more like a descriptive participle phrase and is much less natural here.

Why is hören at the very end, and why isn’t it gehört?

In the Perfekt, German normally uses a past participle (gehört), but in double infinitive constructions (auxiliary + infinitive + infinitive), the perceived-action verb and the perception verb both appear as infinitives:

Ich habe ihn schimpfen hören. (not ... schimpfen gehört)

So the verb cluster at the end is:

schimpfen (infinitive) + hören (infinitive)

What does schimpfen mean here, and how strong is it?

schimpfen usually means to complain, scold, grumble, rant, depending on context. It can be mild (grumbling) or stronger (scolding). In this sentence, it implies you heard him speaking angrily/annoyedly—often something like muttering or ranting about the situation.

What is als doing here, and why not wenn?

als is used for a single event in the past:

als ein Karton vor seiner Tür stand = when a box was standing in front of his door (that one time)

wenn is used for repeated events (whenever) or for present/future conditions:

Wenn ein Karton vor seiner Tür steht, schimpft er. = Whenever a box is in front of his door, he complains.

Why is the verb at the end in the second part: ..., als ... stand?

Because als introduces a subordinate clause, and German subordinate clauses normally send the finite verb to the end:

als ein Karton vor seiner Tür stand (verb stand at the end)

The comma is also required because a subordinate clause is attached to the main clause.

Why is it stand and not stellte?

stand describes a state/position: the box was standing/was there. stellte would mean someone put/placed it there (an action), and it would need a subject like jemand stellte einen Karton ....

So:

Ein Karton stand vor der Tür. = A box was (sitting/standing) in front of the door.
Jemand stellte einen Karton vor die Tür. = Someone placed a box in front of the door.

Why is it vor seiner Tür (dative) and not vor seine Tür (accusative)?

vor is a two-way preposition.

Dative = location (where?) → vor seiner Tür (in front of his door, stationary)
Accusative = movement/direction (where to?) → vor seine Tür (to in front of his door)

Since the carton is already there (stand), it’s location → dative.

Why do we say seiner Tür and not seinem Tür—what gender is Tür?

die Tür is feminine. In dative singular after vor, die becomes der, and sein- becomes seiner:

vor der Tür
vor seiner Tür (= in front of his door)

So seiner matches feminine dative singular.

Could the sentence also be Gestern hörte ich ihn schimpfen ...? Is that more correct?

Yes, it’s correct. That would use Präteritum:

Gestern hörte ich ihn schimpfen, ...

In everyday spoken German, Perfekt (habe ... gehört / habe ... hören) is often preferred, while Präteritum is more common in writing and with certain verbs (especially sein, haben, modals). Both versions are grammatical; the original is very natural in conversation.