Learn Hardcore German: Mein Lernpartner erklärt mir geduldig die Grammatik, wenn ich etwas nicht verstehe. - My study partner patiently explains the grammar to me when I do not understand something.

Since nominative and accusative are the same for feminine nouns, you see die in both cases.
Here it’s accusative because it answers “What is explained?” → die Grammatik.

Why is it mein Lernpartner and not meinen Lernpartner?

Mein Lernpartner is the subject of the sentence, so it is in the nominative case.

The possessive mein declines like ein:

Masculine nominative: mein Lernpartner (subject)
Masculine accusative: meinen Lernpartner (direct object)

In this sentence:

Wer erklärt? (Who explains?) → mein Lernpartner
→ subject → nominative → mein, not meinen.

If you changed the sentence to make Lernpartner the object, you would need meinen:

Ich mag meinen Lernpartner. – I like my learning partner.

Why is geduldig in the middle (erklärt mir geduldig die Grammatik) and can it go somewhere else?

Geduldig is an adverb (patiently) describing how he explains.

In German main clauses, the middle part (between the conjugated verb and the last elements) has a fairly typical order:

Pronoun objects
Adverbs (time, manner, place, etc.)
Noun objects

So:

erklärt (verb, 2nd position)
mir (pronoun object)
geduldig (adverb of manner)
die Grammatik (noun object)

Other positions are possible, but they sound different:

Mein Lernpartner erklärt mir die Grammatik geduldig.
(OK, but a bit less neutral/more end-weighted.)
Mein Lernpartner erklärt geduldig mir die Grammatik.
(Unusual; sounds awkward.)

The original order is the most natural and typical: pronoun → adverb → noun object.

Why is the verb erklärt in the second position and not later, like Mein Lernpartner mir erklärt geduldig …?

German main clauses obey the verb-second (V2) rule:

The conjugated verb must come in second position in a normal statement.

Mein Lernpartner erklärt mir geduldig die Grammatik …

Breakdown:

Mein Lernpartner = first element (subject phrase)
erklärt = conjugated verb in 2nd position
mir geduldig die Grammatik = rest of the clause

You cannot move mir in front of erklärt without breaking the V2 rule:

✗ Mein Lernpartner mir erklärt … → incorrect main-clause word order.
✓ Mein Lernpartner erklärt mir … → correct.

Why is there a comma before wenn, and what does wenn do to the word order?

Wenn introduces a subordinate clause (a dependent clause). In German:

A comma is required before most subordinate clauses.
In a subordinate clause, the conjugated verb goes to the end.

So:

Main clause: Mein Lernpartner erklärt mir geduldig die Grammatik,
Subordinate clause: wenn ich etwas nicht verstehe.

Inside the wenn-clause:

wenn (subordinating conjunction)
ich (subject)
etwas (object)
nicht (negation)
verstehe (conjugated verb at the end)

That’s why we say: wenn ich etwas nicht verstehe, not wenn ich verstehe etwas nicht.

Why is it wenn and not wann here?

Wenn and wann are different:

wenn = when(ever), if
- For conditions, repeated events, and general situations.
- Example: Wenn es regnet, bleibe ich zu Hause. – When it rains, I stay at home.
wann = when (at what time)
- For direct or indirect questions about time.
- Example: Wann kommst du? – When are you coming?
- Ich weiß nicht, wann du kommst. – I don’t know when you’re coming.

In wenn ich etwas nicht verstehe, the meaning is “whenever / when I don’t understand something” – a general, repeated situation, not a question about a specific time. So wenn is correct.

Why is the verb at the end in wenn ich etwas nicht verstehe?

Because wenn is a subordinating conjunction. Such conjunctions (like dass, weil, wenn, obwohl) send the conjugated verb to the end of their clause.

Word order in the wenn-clause:

wenn (subordinating conjunction)
ich (subject)
etwas (object)
nicht (negation)
verstehe (conjugated verb at the end)

So the pattern is:

wenn + subject + (objects/other info) + verb-final

Example with the same pattern:

Wenn ich Zeit habe, helfe ich dir.
(If/When I have time, I help you.)
→ habe at the end of the wenn-clause.

Why is the negation nicht placed before verstehe and after etwas?

General rule in such clauses:

Subject → objects → nicht → verb (at the end in subordinate clauses)

In wenn ich etwas nicht verstehe:

ich = subject
etwas = object
nicht = negation of the verb phrase
verstehe = verb (final position)

You normally say:

Ich verstehe etwas nicht.
but
Ich verstehe nicht. (when there’s no object)

Putting nicht directly before the verb and after the object is the most neutral way to negate the whole statement:

ich [etwas nicht verstehe] → I don’t understand something.

Why is it etwas and not was for “something”?

In standard German:

etwas = something (indefinite pronoun)
was = what (question word) or a relative pronoun

So the most neutral form is:

wenn ich etwas nicht verstehe = when I don’t understand something.

In colloquial spoken German, people often use was instead of etwas:

wenn ich was nicht verstehe

This is very common in conversation, but etwas is the more standard and formal form, and it’s what you usually learn first.

Why is the present tense (erklärt, verstehe) used, even though this can be about future situations too?

German uses the present tense very often, including for:

Habitual actions: things that happen regularly
General truths and typical behavior
Many future situations, when the time is clear from context or from another expression

The sentence describes a repeated, typical situation:

Mein Lernpartner erklärt mir …, wenn ich etwas nicht verstehe.
→ My learning partner explains to me when(ever) I don’t understand something.

You could form a future:

… wird mir erklären …, wenn ich etwas nicht verstehen werde.

But this sounds very unnatural here. The simple present is the normal, idiomatic choice.

Why is Lernpartner written as one word, and how does that affect gender?

German loves compound nouns. They are usually written as one long word.

Lernpartner is a compound:

Lern- (from lernen / das Lernen, learning)
Partner (partner)

Rules for compounds:

They are written as one word: Lernpartner, not Lern Partner.
The last part of the compound decides:
- the grammatical gender
- the plural form

Here, the last part is Partner:

der Partner → masculine
So der Lernpartner is also masculine.

Plural:

der Partner → die Partner
der Lernpartner → die Lernpartner

Why is it ich and not mich in wenn ich etwas nicht verstehe?

Ich is the nominative form of the pronoun (subject form):

Nominative: ich = I (subject)
Accusative: mich = me (direct object)
Dative: mir = to me / for me (indirect object)

In the wenn-clause, ich is the subject of the verb verstehe:

Wer versteht (etwas nicht)? → ich

So you need the subject form → ich, not mich.

Why are Grammatik and Lernpartner capitalized?

In German, all nouns are capitalized, regardless of where they appear in the sentence.

die Grammatik – noun → capital G
der Lernpartner – noun → capital L

This also applies to compound nouns like Lernpartner: the whole compound is written with an initial capital letter.

Can I start the sentence with the wenn-clause instead, and how does that change the word order?

Yes, you can put the wenn-clause first:

Wenn ich etwas nicht verstehe, erklärt mir mein Lernpartner geduldig die Grammatik.

Word order rules:

The entire wenn-clause counts as one “element” in front position.
In the main clause that follows, the finite verb must still be in second position.

So after the comma:

erklärt must come first (2nd element of the whole sentence),
then the subject and the rest: mir mein Lernpartner geduldig die Grammatik or more naturally mein Lernpartner mir geduldig die Grammatik.

Both orders are possible:

Wenn ich etwas nicht verstehe, erklärt mir mein Lernpartner geduldig die Grammatik.
Wenn ich etwas nicht verstehe, erklärt mein Lernpartner mir geduldig die Grammatik.

Starting with wenn adds a little emphasis on the condition (“Whenever I don’t understand something…”).