Learn Hardcore German: Mein Bruder isst heimlich Schokolade in der Küche, wenn er Stress hat. - My brother secretly eats chocolate in the kitchen when he is stressed.

In this sentence you need essen, because the brother is eating chocolate, not just existing.
So: Mein Bruder isst heimlich Schokolade … = My brother eats chocolate …

Why is there no article before Schokolade? Why not eine Schokolade?

In German, some nouns often work like mass/uncountable nouns in English:

Schokolade can mean chocolate in general, not one single chocolate bar.
When you mean an indefinite, uncountable amount, you normally leave out the article:
- Ich trinke Kaffee. = I drink coffee.
- Er isst Schokolade. = He eats chocolate.

You could say eine Schokolade or eine Tafel Schokolade, but that would refer to a specific item (for example one bar or one piece), not the general habit or substance.

Why is it in der Küche and not in die Küche?

The preposition in can take either dative or accusative:

Dative → location (where something is)
Accusative → movement (where something is going to)

In this sentence, the brother is already in the kitchen while he eats:

in der Küche (dative) = in the kitchen (location)
in die Küche (accusative) = into the kitchen (direction, movement)

Compare:

Er isst in der Küche. = He eats in the kitchen.
Er geht in die Küche. = He goes into the kitchen.

Why is it der Küche in the dative? I thought die Küche is feminine.

You are right: the basic form is die Küche (feminine).

In the dative singular, all feminine nouns take the article der:

Nominative: die Küche (the kitchen)
Dative: in der Küche (in the kitchen)

So der here is the dative feminine article, not the masculine nominative article. Same pattern:

die Stadt → in der Stadt
die Schule → in der Schule

Why does heimlich come right after isst? Could it go somewhere else?

heimlich is an adverb (secretly). German adverbs are relatively flexible in position.

The original sentence:

Mein Bruder isst heimlich Schokolade in der Küche …

Other possible positions (all correct, with small differences in emphasis):

Mein Bruder isst Schokolade heimlich in der Küche …
Mein Bruder isst in der Küche heimlich Schokolade …

What you usually avoid is splitting the verb from its object too strangely, like:

? Mein Bruder isst in der Küche Schokolade heimlich. (understandable, but sounds odd)

The version with isst heimlich Schokolade sounds very natural and smoothly puts the focus on the secretly eating.

Why is there a comma before wenn, and why does the verb go to the end in wenn er Stress hat?

wenn introduces a subordinate clause in German. Two rules apply:

A subordinate clause is separated from the main clause with a comma:
- …, wenn er Stress hat.
In many subordinate clauses (including those with wenn), the finite verb goes to the end:
- er hat Stress → wenn er Stress hat

So the sentence structure is:

Main clause (verb in 2nd position):
Mein Bruder isst heimlich Schokolade in der Küche, …
Subordinate clause (verb at the end):
… wenn er Stress hat.

Why is it wenn and not als here?

Both wenn and als can be translated as when, but they are used differently:

wenn is used for:
- repeated events
- general conditions
- present and future time
als is used for:
- a single, specific event in the past

Here, the sentence describes a general habit: whenever he is stressed, he secretly eats chocolate. That is a repeated / general situation → you must use wenn:

…, wenn er Stress hat. = whenever he is stressed

You would use als in something like:

Als er gestern Stress hatte, ist er in die Küche gegangen.
When he was stressed yesterday, he went to the kitchen.
(one specific past incident)

Why is the word order er Stress hat and not er hat Stress?

In a main clause, German is typically Subject – Verb – Object:

Er hat Stress. = He is stressed / He has stress.

In a subordinate clause introduced by wenn, the finite verb moves to the end of the clause:

wenn
- er
  - Stress
    - hat

So it changes from:

main clause: er hat Stress
to
subordinate clause: wenn er Stress hat

Could I also say wenn er gestresst ist instead of wenn er Stress hat? Is there a difference?

Yes, you can say both; they are very close in meaning:

wenn er Stress hat = literally when he has stress
wenn er gestresst ist = when he is stressed

Nuances:

Stress haben can suggest that there are many tasks, pressure, deadlines, etc. (a stressful situation).
gestresst sein focuses more on his emotional/mental state (he is feeling stressed).

In everyday language, they often overlap. In this context of comfort-eating chocolate, both sound natural.

Why is er used in the second part, and not mein Bruder again?

In German, once a noun has been mentioned, you normally replace it with a pronoun later to avoid repetition, just like in English.

First mention: Mein Bruder … (my brother)
Later reference: er … (he)

So in wenn er Stress hat, er clearly refers to mein Bruder. Using mein Bruder again would sound repetitive and less natural:

Mein Bruder isst …, wenn mein Bruder Stress hat. (grammatically correct, but stylistically poor)

Why is the present tense used here? In English, we’d also say eats for a habit, is it the same in German?

Yes, it’s very similar:

German present tense (Präsens) is used for:
- actions happening now
- general truths
- repeated actions and habits

So:

Mein Bruder isst heimlich Schokolade, wenn er Stress hat.
= My brother secretly eats chocolate when he is stressed.
(habitual behavior)

You sometimes see present tense in German where English might use something else (like future), but for habits it works in a very parallel way.