Wir klatschen beim Refrain mit.

Because the verb here is the separable verb mitklatschen.

• Full verb: mitklatschen = to clap along
• In a normal main clause, the prefix mit- separates and goes to the end:
  • Wir klatschen ... mit.

This is very common in German:

• Ich mache mit. = I join in.
• Sie singt mit. = She sings along.
• Wir klatschen mit. = We clap along.

If the verb is not split, for example in the infinitive, it stays together:

• Wir wollen mitklatschen.

Not quite. klatschen means to clap, but mitklatschen means to clap along / clap with others / join in the clapping.

So mit- adds the idea of participating together with other people.

Compare:

• Wir klatschen. = We clap.
• Wir klatschen mit. = We clap along / join in clapping.

That little mit changes the feeling of the sentence quite a bit.

beim is a contraction of bei dem.

• bei
  • dem = beim

So:

• beim Refrain = literally at the chorus
• in natural English here, it usually means during the chorus or when the chorus comes

This is a very common contraction in German, just like:

• im = in dem
• am = an dem
• zum = zu dem

Because the preposition bei takes the dative case.

The noun is:

• der Refrain = the chorus

In the dative singular, der becomes dem:

• bei dem Refrain
• contracted: beim Refrain

So the structure is:

• bei
  • dative noun phrase

Usually beim Refrain is the more natural choice here.

• beim Refrain means something like during the chorus / when the chorus comes
• im Refrain means more literally in the chorus, as part of the song itself

So:

• Wir klatschen beim Refrain mit. = We clap along during the chorus.
• Im Refrain would sound more like you are talking about something located inside the chorus section of the song.

Both can exist in German, but they do not feel exactly the same.

Yes, you can, but the meaning changes slightly.

• Wir klatschen beim Refrain. = We clap during the chorus.
• Wir klatschen beim Refrain mit. = We clap along during the chorus.

With mit, the sentence clearly suggests joining in with others. Without it, it just says that the clapping happens at that time.

No. klatschen can mean to clap in different situations.

For example:

• applaudieren = to applaud
• klatschen = to clap
• mitklatschen = to clap along

In a music context, klatschen often means clapping rhythmically, not necessarily formal applause at the end of a performance.

So in this sentence, it sounds like people are joining in with the rhythm during the chorus.

klatschen is in the present tense: wir klatschen = we clap / we are clapping.

Like the English present, German present tense can cover more than one idea depending on context:

• We clap along during the chorus (habitual / general)
• We are clapping along during the chorus (right now)

Without more context, the sentence could be understood either way.

The sentence follows the normal German main clause pattern:

• Wir = subject
• klatschen = conjugated verb in second position
• beim Refrain = time/context phrase
• mit = separated prefix at the end

So the pattern is:

Subject + finite verb + other elements + separable prefix

That is why the sentence is:

• Wir klatschen beim Refrain mit.

and not:

• Wir mit klatschen beim Refrain. ❌

Yes, it matters because it affects the article.

• der Refrain = the chorus

Since Refrain is masculine, and bei requires the dative, you get:

• bei dem Refrain
• beim Refrain

If the noun had a different gender, the form would change:

• bei der ... for feminine
• bei dem ... / beim ... for masculine or neuter singular

So knowing the gender helps you build the correct case form.