Meine Nachhilfelehrerin sagt, ein Nebensatz sei leichter, wenn man zuerst den Hauptsatz versteht.

Sei is the Konjunktiv I form of sein. German often uses Konjunktiv I for reported speech or indirect speech.

So here:

• Meine Nachhilfelehrerin sagt ... = My tutor says ...
• ein Nebensatz sei leichter = that a subordinate clause is easier, as a reported statement

Using sei shows that the speaker is reporting what the tutor says, rather than simply stating it as a fact in their own voice.

Compare:

• Ein Nebensatz ist leichter. = direct statement
• Meine Nachhilfelehrerin sagt, ein Nebensatz sei leichter. = reported speech

In everyday spoken German, many people would also say ist here, especially with dass:

• Meine Nachhilfelehrerin sagt, dass ein Nebensatz leichter ist ...

But sei is very normal in careful written German and in formal reported speech.

German can report speech in two common ways:

1. with dass

   • Meine Nachhilfelehrerin sagt, dass ein Nebensatz leichter ist/sei.

2. without dass, often with Konjunktiv I

   • Meine Nachhilfelehrerin sagt, ein Nebensatz sei leichter.

The sentence you have uses the second pattern. This is a standard way to express indirect speech.

A useful point:

• If dass is present, the verb usually goes to the end of that clause.
• If dass is omitted in this kind of reported speech, the clause keeps normal main-clause word order.

So:

• ..., dass ein Nebensatz leichter sei
• ..., ein Nebensatz sei leichter

Both are possible.

Because wenn introduces a subordinate clause. In German subordinate clauses, the finite verb goes to the end.

So:

• wenn man zuerst den Hauptsatz versteht

Breakdown:

• wenn = if/when
• man = one / you / people
• zuerst = first
• den Hauptsatz = the main clause
• versteht = understands

This verb-final position is one of the most important patterns in German.

Compare:

• Main clause: Man versteht den Hauptsatz zuerst.
• Subordinate clause: ..., wenn man zuerst den Hauptsatz versteht.

It can mean either if or when, depending on context.

Here, the meaning is something like a general condition, so English often translates it best as if:

• A subordinate clause is easier if you first understand the main clause.

But when is also possible in a more general sense:

• ... when you first understand the main clause.

In many grammar explanations like this, English if usually sounds more natural.

No. German man does not mean an adult male here.

It is an impersonal pronoun, meaning:

• one
• you in a general sense
• people

So:

• wenn man zuerst den Hauptsatz versteht means
• if one first understands the main clause
• if you first understand the main clause

In natural English, you is often the best translation, even though German uses man.

Because they have different grammatical roles.

ein Nebensatz

This is the subject of sei:

• ein Nebensatz sei leichter
• a subordinate clause is easier

As the subject, it is in the nominative case.

den Hauptsatz

This is the direct object of versteht:

• man versteht den Hauptsatz
• one understands the main clause

The verb verstehen takes the accusative, so der Hauptsatz becomes den Hauptsatz.

So the contrast is:

• ein Nebensatz = nominative subject
• den Hauptsatz = accusative object

Leichter is the comparative form of leicht:

• leicht = easy / light
• leichter = easier

Here it is used predicatively, after the verb sein:

• ein Nebensatz sei leichter

In that position, adjectives normally do not take adjective endings.

Compare:

• ein leichter Satz = an easy sentence
  → adjective before a noun, so it takes an ending

• der Satz ist leicht = the sentence is easy
  → adjective after sein, so no ending

So leichter here just means easier.

It means female tutor or female private teacher.

German often builds long words by combining smaller words into compound nouns.

Here:

• Nachhilfe = tutoring / extra help with schoolwork
• Lehrerin = female teacher

Together:

• Nachhilfelehrerin = female tutor

A few useful points:

• The last part usually determines the basic meaning and grammatical gender.
• Since Lehrerin is feminine, Nachhilfelehrerin is also feminine.

Related forms:

• der Lehrer = male teacher
• die Lehrerin = female teacher
• der Nachhilfelehrer = male tutor
• die Nachhilfelehrerin = female tutor

These are grammar terms.

• der Hauptsatz = main clause
• der Nebensatz = subordinate clause

A main clause can stand on its own as a complete sentence.

Example:

• Ich verstehe den Hauptsatz.
• I understand the main clause.

A subordinate clause depends on another clause and is often introduced by words like:

• dass
• wenn
• weil
• ob

In German, subordinate clauses usually send the finite verb to the end.

So this sentence is talking about grammar using grammar terms:

• the tutor says a subordinate clause is easier if you first understand the main clause.

It has three parts:

1. Meine Nachhilfelehrerin sagt

   • main clause
   • My tutor says

2. ein Nebensatz sei leichter

   • reported statement in indirect speech
   • a subordinate clause is easier

3. wenn man zuerst den Hauptsatz versteht

   • wenn-clause giving the condition
   • if one first understands the main clause

So the overall structure is:

• [main clause] + [reported statement] + [subordinate wenn-clause]

The commas help separate these clause boundaries.

Yes. A more everyday version would often be:

• Meine Nachhilfelehrerin sagt, dass ein Nebensatz leichter ist, wenn man zuerst den Hauptsatz versteht.

Or even:

• Meine Nachhilfelehrerin sagt, ein Nebensatz ist leichter, wenn man zuerst den Hauptsatz versteht.

These sound more conversational to many learners.

The version with sei is especially useful to know because:

• it is grammatically correct,
• it appears in formal writing,
• and it teaches you how German handles indirect speech.

So the original sentence is not strange—it is just a bit more grammatical/formal in style.