Im Kurs verwechsle ich manchmal die Einzahl mit der Mehrzahl.

German often puts a time or place expression first to set the scene. Here, Im Kurs means in class or during the course, and it is placed first for emphasis or simply as a natural sentence opener.

Even though Im Kurs comes first, it counts as the first element of the sentence, so the conjugated verb must still come second.

Im is a contraction of in dem.

• in dem Kurs → im Kurs

This is very common in German. In this sentence, im Kurs is idiomatic and natural.

This is because German main clauses usually follow the verb-second rule.

The first element here is Im Kurs, so the conjugated verb must come next:

• Im Kurs = position 1
• verwechsle = position 2
• ich comes after that

If the sentence started with Ich, then it would be:

• Ich verwechsle im Kurs manchmal die Einzahl mit der Mehrzahl.

Both are correct.

The verb verwechseln means to mix up or to confuse.

A very common pattern is:

• etwas mit etwas verwechseln = to confuse one thing with another

So in this sentence:

• die Einzahl = the thing being confused
• mit der Mehrzahl = what it is being confused with

This pattern is important because it affects the cases used.

Because the two nouns are in different cases.

In die Einzahl mit der Mehrzahl:

• die Einzahl is the direct object of verwechsle, so it is in the accusative
• der Mehrzahl comes after mit, and mit always takes the dative

Both Einzahl and Mehrzahl are feminine nouns.

So:

• nominative/accusative feminine: die
• dative feminine: der

That is why the articles are different.

Yes. Mit is a dative preposition.

So whenever you use mit, the following noun phrase must be in the dative case.

Examples:

• mit dem Lehrer
• mit der Mehrzahl
• mit den Kindern

That is why the sentence has mit der Mehrzahl, not mit die Mehrzahl.

They are the standard German grammar terms for:

• Einzahl = singular
• Mehrzahl = plural

You may also sometimes see the Latin-based terms:

• Singular
• Plural

But Einzahl and Mehrzahl are very common and completely standard in German.

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

That includes grammar terms like:

• der Singular
• der Plural
• die Einzahl
• die Mehrzahl

So the capitalization is not optional.

Manchmal means sometimes. It is an adverb, and German allows some flexibility with its placement.

In this sentence, manchmal appears in the middle:

• Im Kurs verwechsle ich manchmal die Einzahl mit der Mehrzahl.

That sounds natural.

You could also say:

• Ich verwechsle manchmal im Kurs die Einzahl mit der Mehrzahl.
• Manchmal verwechsle ich im Kurs die Einzahl mit der Mehrzahl.

These versions are also grammatical, but they shift the emphasis slightly.

No. Verwechseln is not separable.

So you say:

• Ich verwechsle das.
• Ich habe das verwechselt.

You do not split it as something like wechsle ... ver.

This is worth remembering because many German verbs with prefixes are separable, but ver- verbs usually are not.

It can mean either, depending on context.

• in the course = during the course
• in class = in the classroom lesson or class setting

In everyday English, in class is often the most natural translation if someone is talking about making grammar mistakes while learning. But the exact nuance depends on the situation.

Yes. That is also correct.

German word order is flexible as long as the basic grammar rules are respected. Your version places manchmal later and keeps Ich first:

• Ich verwechsle die Einzahl manchmal mit der Mehrzahl.

This is grammatical, though many speakers might prefer:

• Ich verwechsle manchmal die Einzahl mit der Mehrzahl.

Both are natural. The difference is mostly one of emphasis and rhythm.