Im Alltag verwechsle ich den Dativ noch manchmal mit dem Akkusativ, aber im Kurs passiert das seltener.

German main clauses use the V2 rule: the finite verb must be in the second position.

So in:

Im Alltag verwechsle ich ...

the first position is the whole phrase Im Alltag, and the second position must be the verb verwechsle. That pushes ich after the verb.

You could also say:

Ich verwechsle im Alltag ...

That is also correct, but it gives a slightly different emphasis. Starting with Im Alltag highlights the everyday-life context.

im is a contraction of in dem.

• im Alltag = in everyday life
• im Kurs = in the course / in class

German often contracts in dem to im, especially in common expressions like these.

So:

• in dem Alltag → usually im Alltag
• in dem Kurs → usually im Kurs

This is a very common point of confusion, and it has to do with the structure of verwechseln.

The pattern is:

etwas/jemanden mit etwas/jemandem verwechseln
= to confuse something/someone with something/someone

So here:

• den Dativ is the direct object of verwechseln, so it is in the accusative
• mit dem Akkusativ is part of a mit-phrase, and mit always takes the dative

That means:

• der Dativ → den Dativ because it is the direct object
• der Akkusativ → dem Akkusativ because it follows mit

Important: Akkusativ here is just the name of a grammar case. In mit dem Akkusativ, the word Akkusativ itself is in the dative form because of mit.

Because in German, Dativ and Akkusativ are ordinary nouns when you are talking about the names of the cases.

They are masculine nouns:

• der Dativ
• der Akkusativ

And like other nouns, they take articles and change by case:

• nominative: der Dativ
• accusative: den Dativ
• dative: dem Dativ

Same idea for Akkusativ.

Because they are nouns, and all nouns are capitalized in German.

So:

• der Dativ
• der Akkusativ
• der Kurs
• der Alltag

all begin with capital letters.

manchmal means sometimes.

Adding noch gives the sense of still or even now:

• manchmal = sometimes
• noch manchmal = still sometimes

So noch suggests that this mistake continues to happen, but perhaps less than before. It often implies progress:

I still sometimes confuse the dative with the accusative.

verwechseln means to confuse / mix up.

The usual pattern is:

A mit B verwechseln
= to confuse A with B

In this sentence:

• A = den Dativ
• B = dem Akkusativ

So literally the structure is:

I confuse the dative with the accusative.

This is the normal and natural pattern in German.

German often uses das to refer back to a whole previous idea.

Here, das refers to the action of confusing the cases:

... aber im Kurs passiert das seltener.
= ... but in class that happens less often.

So das does not refer to a single noun like der Dativ or der Akkusativ. It refers to the whole situation.

Here das is in the nominative, because it is the subject of passiert.

Think of it like:

Das passiert.
= That happens.

So in:

im Kurs passiert das seltener

the thing that happens less often is das.

seltener is the comparative of selten:

• selten = seldom / rarely
• seltener = less often / more rarely

So passiert das seltener means that happens less often.

You could also say weniger oft, and the meaning would be very similar. seltener is just a bit more compact and natural here.

There is no als because the comparison is only implied, not fully stated. The sentence means:

... less often than in everyday life

but German does not need to say that explicitly if the comparison is clear from context.

They describe two different situations:

• im Alltag = in everyday life, outside the classroom
• im Kurs = in the course, in class, during lessons

So the speaker is contrasting:

• how often the mistake happens in normal daily use
• how often it happens in a learning environment

This contrast is reinforced by aber.

Yes. German word order is flexible as long as the main grammar rules are respected.

For example, these are possible:

Ich verwechsle im Alltag den Dativ noch manchmal mit dem Akkusativ, aber im Kurs passiert das seltener.

or

Den Dativ verwechsle ich im Alltag noch manchmal mit dem Akkusativ, aber im Kurs passiert das seltener.

These versions change the emphasis:

• Im Alltag ... emphasizes the context
• Ich ... emphasizes the speaker
• Den Dativ ... emphasizes what is being confused

The original sentence sounds very natural because it starts by setting the scene: In everyday life...