Wenn der Stich juckt, kratze ich lieber nicht daran.

Here wenn means if or when(ever) in the sense of a condition or repeated situation:

• Wenn der Stich juckt ... = If/Whenever the bite/sting itches ...

Important differences:

• wenn = if, or when for repeated/general situations
• wann = when in a question
  
  • Wann juckt der Stich? = When does the bite itch?
• als = when for a single event in the past
  
  • Als der Stich gejuckt hat ... = When the bite was itching ...

So in this sentence, wenn is the natural choice because it describes a condition: if that happens, this is what I do.

Because wenn introduces a subordinate clause. In German, subordinate clauses send the conjugated verb to the end.

So:

• der Stich juckt = main clause word order
• wenn der Stich juckt = subordinate clause, with juckt at the end

This is a very common pattern:

• Wenn ich Zeit habe, komme ich.
• Weil es regnet, bleiben wir zu Hause.

So Wenn der Stich juckt follows normal German subordinate-clause word order.

Because the sentence begins with the wenn-clause, and that whole clause counts as the first element of the sentence. In German main clauses, the conjugated verb must come in second position.

Structure:

• Wenn der Stich juckt, = first element
• kratze = verb in second position
• ich = subject after the verb

So:

• Wenn der Stich juckt, kratze ich lieber nicht daran.

This is standard German word order. Compare:

• Ich kratze lieber nicht daran, wenn der Stich juckt.
• Wenn der Stich juckt, kratze ich lieber nicht daran.

Both are possible, but in the second one the verb has to come before ich.

Der Stich can mean several things depending on context, but here it most naturally means something like:

• an insect bite
• an insect sting
• sometimes more generally a prick or puncture

Since the sentence mentions itching and scratching, a learner will usually understand it as something like a mosquito bite or bug bite/sting.

A few related words:

• der Biss = a bite, especially from an animal
• der Stich = a sting, prick, or bite from an insect / needle / sharp object

In everyday speech, Stich is often used for the irritated spot itself.

Because the subject is der Stich, which is third person singular.

The verb is jucken = to itch, and its present-tense forms include:

• ich jucke (rarely used this way in everyday speech)
• du juckst
• er/sie/es juckt
• wir jucken
• ihr juckt
• sie/Sie jucken

Since der Stich = he/it grammatically, the correct form is:

• Der Stich juckt.

Lieber means rather or preferably. In this sentence:

• Ich kratze lieber nicht daran.
• = I’d rather not scratch it.

So lieber nicht expresses a preference to avoid the action.

It does not simply mean a plain negative like I do not scratch it. It adds the idea:

• I prefer not to
• I’d rather not

That makes the sentence sound more natural and nuanced.

Because lieber nicht works together as a common expression meaning rather not.

• Ich kratze lieber nicht daran. = I’d rather not scratch it.

If you move nicht, the sentence may sound less natural or the emphasis may change.

For learners, it is best to remember lieber nicht as a set phrase:

• Ich gehe lieber nicht. = I’d rather not go.
• Wir sagen lieber nichts. = We’d rather not say anything.

So in this sentence, lieber nicht is the most natural order.

Daran means at it / on it / to it, depending on the verb. Here it goes with kratzen an:

• an etwas kratzen = to scratch at something

So:

• daran kratzen = scratch at it

German often uses da- compounds when referring back to a thing:

• daran = an + das/it
• darauf = on it
• damit = with it
• darüber = about it

This is very common when the thing is not a person.

Because the verb here is not just kratzen in a direct-object sense. It is an etwas kratzen.

That means the sentence needs the prepositional element an:

• an dem Stich kratzen
• daran kratzen

You cannot usually replace that with a simple es, because es would suggest a direct object:

• Ich kratze es. — possible only in a different structure, and not the most natural choice here
• Ich kratze daran. — natural for scratch at it

So daran is there because the verb pattern requires an.

No. Kratzen can be used in more than one way.

1. Intransitive / with a preposition

   • an etwas kratzen = to scratch at something
   • Er kratzt an der Tür. = He is scratching at the door.

2. Reflexive

   • sich kratzen = to scratch oneself
   • Ich kratze mich. = I scratch myself.

3. Direct object

   • sometimes etwas kratzen, depending on context

In this sentence, daran kratzen sounds natural because the idea is scratching at the itchy spot.

Because in German, a subordinate clause introduced by wenn is separated from the main clause with a comma.

So:

• Wenn der Stich juckt, kratze ich lieber nicht daran.

This comma is not optional. German punctuation requires it.

You will see the same pattern with many conjunctions:

• Weil ich müde bin, gehe ich schlafen.
• Obwohl es regnet, gehen wir spazieren.
• Wenn du willst, komme ich mit.