Learn Hardcore German: Ich reiche ihr ein Taschentuch, wenn sie niest, und frage, ob die Erkältung schon besser ist. - I hand her a tissue when she sneezes and ask whether the cold is already better.

If you said ich reiche sie ein Taschentuch, sie would be accusative (her) instead of dative (to her), so it would be grammatically wrong. The person after reichen must be in the dative: ich reiche ihr ….

Why is it ein Taschentuch and not einen Taschentuch?

Because Taschentuch is neuter in German:

das Taschentuch = the tissue / handkerchief

In the accusative singular, the indefinite article is:

masculine: einen Mann
feminine: eine Frau
neuter: ein Taschentuch

Here, ein Taschentuch is the direct object (accusative) of reiche, and since Taschentuch is neuter, we must use ein, not einen.

Word order rule also fits: (Dative) – (Accusative)

Ich reiche ihr (DAT) ein Taschentuch (AKK).

What is the difference between reichen and geben here? Can I say ich gebe ihr ein Taschentuch instead?

You can say ich gebe ihr ein Taschentuch – it is perfectly correct.

The nuance:

geben = to give (neutral, the standard word)
reichen = to hand / pass (often with the idea of physically handing something over, sometimes a bit more polite or formal)

Examples:

Kannst du mir bitte das Salz reichen? – Can you pass me the salt, please?
Der Kellner reichte ihr die Speisekarte. – The waiter handed her the menu.

In your sentence, reichen paints the picture of actually handing the tissue to her at the moment she sneezes. geben would sound a bit more neutral and less “gesture-focused”, but both are fine.

What exactly does wenn sie niest mean – is it “when she sneezes” or “if she sneezes”?

Wenn in German can mean both:

when / whenever (repeated or general events)
if (conditional)

In this sentence, wenn sie niest most naturally means:

whenever she sneezes or
when she sneezes (in general / habitually)

So the whole sentence describes a habitual action:

Ich reiche ihr ein Taschentuch, wenn sie niest …
→ Whenever she sneezes, I hand her a tissue…

The present tense in both clauses (reiche, niest) expresses a regular, repeated situation, just like in English:
When she sneezes, I give her a tissue.

Why is it wenn here and not als or wann?

These three words are easy to confuse:

wenn
- when/whenever in the present, future, or for repeated events
- if (conditional)
als
- when for a single event in the past
- e.g. Als ich klein war, spielte ich viel draußen.
wann
- when? as a question word
- used in direct or indirect questions about time
- e.g. Wann kommst du? / Ich weiß nicht, wann er kommt.

In your sentence, we are talking about a general, repeated situation (she sometimes sneezes, whenever she does, I hand her a tissue), so:

wenn sie niest is correct.
als sie niest – wrong (and tense mismatch; als is for one-time past events).
wann sie niest – wrong here, because we are not asking at what time she sneezes.

Why are there commas around wenn sie niest?

The sentence is:

Ich reiche ihr ein Taschentuch, wenn sie niest, und frage, ob die Erkältung schon besser ist.

Structure:

Main clause: Ich reiche ihr ein Taschentuch
Subordinate clause: wenn sie niest
Continuation of main clause: und frage …

German rules:

Every subordinate clause is separated by a comma from the main clause.
- So you need a comma before wenn sie niest.
If the subordinate clause is inserted in the middle of a larger main clause, it is normally surrounded by commas.
- That’s why there is also a comma after niest, before und.

So wenn sie niest is like a small block inserted into the main sentence, and German marks that block with commas on both sides.

Why is there no ich before frage? Why not …, und ich frage, ob …?

German often omits the subject in the second clause if it’s the same subject as in the first clause and the two clauses are joined by und or oder.

Compare:

Ich gehe nach Hause und schlafe.
(Subject ich applies to both gehe and schlafe.)
Full form: Ich gehe nach Hause und ich schlafe. (Possible, but usually only for emphasis or clarity.)

In your sentence, the subject ich is understood for both verbs:

Ich reiche ihr ein Taschentuch, … und (ich) frage, ob …

You could say …, und ich frage, ob …, and that is grammatically correct. Leaving out the second ich is just more natural and less repetitive.

Why is it ob die Erkältung schon besser ist and not wenn die Erkältung schon besser ist?

Ob is used for indirect yes/no questions (whether-questions), especially after verbs like fragen, wissen, nicht wissen, sich fragen, etc.

Pattern:

Ich frage, ob … – I ask whether / if …
Ich weiß nicht, ob … – I don’t know whether / if …

Here, frage, ob die Erkältung schon besser ist means:

I ask whether the cold is already better.

You cannot use wenn in this indirect question. Wenn would make it a conditional clause:

Wenn die Erkältung schon besser ist, …
→ If the cold is already better, then …

That is a different meaning. So after frage (ask), you must use ob for “whether / if”.

Why is the verb at the end in ob die Erkältung schon besser ist? Why not ob die Erkältung ist schon besser?

In subordinate clauses introduced by words like dass, weil, wenn, ob, the finite verb normally goes to the end of the clause.

Compare:

Main clause:
Die Erkältung ist schon besser.
(Verb in 2nd position.)
Subordinate clause:
… ob die Erkältung schon besser ist.
(Verb ist at the end.)

So the correct structure is:

ob (subordinating conjunction)
die Erkältung (subject)
schon besser (adverb + adjective)
ist (finite verb at the end)

*ob die Erkältung ist schon besser is ungrammatical, because it uses main-clause word order inside a subordinate clause.

What does schon add in schon besser? Is it necessary?

Schon here has the meaning of already / by now.

Ist die Erkältung besser?
→ Is the cold better?
Ist die Erkältung schon besser?
→ Is the cold better already / yet? (Is there improvement by now?)

Nuance:

Without schon: neutral question about improvement.
With schon: you imply some expectation of improvement or you’re checking whether it has improved by this point in time.

It is not grammatically required, but it adds this time-related nuance. The placement is standard:

schon usually comes before the adjective or adverb it relates to:
schon besser, not besser schon.

Why is Erkältung capitalized, but besser and schon are not?

In German, all nouns are capitalized, regardless of whether they are common or proper nouns.

die Erkältung – noun → capitalized
besser – adjective → lower case
schon – adverb/particle → lower case

So Erkältung is written with a capital E because it’s a noun (meaning cold, as in illness), and this follows the general rule:
All nouns in German start with a capital letter.

Why do we say die Erkältung? Could we leave out the article and just say ob Erkältung schon besser ist?

We say die Erkältung because:

Erkältung is feminine:
- die Erkältung (the cold)
Here we are talking about a specific, known cold (the one she currently has), not about colds in general.

In German, a specific countable noun normally needs an article (definite or indefinite). So:

ob die Erkältung schon besser ist – correct
*ob Erkältung schon besser ist – sounds wrong/unnatural in standard German

You could leave out the article only in certain set expressions or with plural / mass nouns, but not here with a specific singular countable noun like Erkältung.