Das Schulprojekt, das wir im Unterricht planen, ist interessant.

The two das words do different jobs:

• The first das is the definite article for Schulprojekt:
  das Schulprojekt = the school project
• The second das is a relative pronoun:
  das wir im Unterricht planen = that we are planning in class

So they look the same, but they are not the same part of speech.

Also, the second das is in the accusative case, because it is the thing being planned:

• wir planen das Schulprojekt

That is why the relative pronoun is das here.

It is a relative clause. A relative clause gives more information about a noun.

Here, it gives more information about das Schulprojekt:

• main noun: das Schulprojekt
• extra information: das wir im Unterricht planen

So the full idea is:

• The school project — which we are planning in class — is interesting.

The relative clause is inserted into the middle of the main sentence.

Because in German, relative clauses are separated by commas.

So:

• Das Schulprojekt, das wir im Unterricht planen, ist interessant.

This is required in standard German spelling. English often uses commas less strictly, but German is much more consistent about this.

Because a relative clause is a type of subordinate clause, and in German subordinate clauses, the conjugated verb goes to the end.

Compare:

• main clause: Wir planen das Schulprojekt im Unterricht.
• relative clause: das wir im Unterricht planen

So when the clause becomes subordinate, planen moves to the end.

This is one of the most important word-order rules in German.

The basic main clause is:

• Das Schulprojekt ist interessant.

Then German inserts the relative clause right after Schulprojekt:

• Das Schulprojekt, das wir im Unterricht planen, ist interessant.

So ist is still the verb of the main clause, but the relative clause comes in between the subject and the verb.

This can feel unusual to English speakers, but it is very common in German.

Because the relative pronoun must match the noun it refers to in gender and number, but its case depends on its role inside the relative clause.

The noun is:

• das Schulprojekt → neuter singular

So the relative pronoun must be neuter singular too.

Inside the relative clause, it is the direct object of planen:

• wir planen das Schulprojekt

That means it must be accusative neuter singular, which is also das.

So:

• gender/number comes from Schulprojekt
• case comes from its function in the clause

im is a contraction of:

• in dem → im

So:

• im Unterricht = in the lesson / in class

It uses dem because Unterricht is masculine:

• der Unterricht

And after in for a location, German often uses the dative case:

• in dem Unterricht → im Unterricht

So this means during class / in class.

Because interessant is being used as a predicate adjective, after the verb sein.

In German:

• attributive adjective before a noun: ein interessantes Projekt
• predicate adjective after sein: Das Projekt ist interessant

Predicate adjectives do not take adjective endings.

So here:

• Das Schulprojekt ist interessant.

not:

• Das Schulprojekt ist interessante. ❌

Schulprojekt is a compound noun, which is very common in German.

It is made from:

• Schule = school
• Projekt = project

Together:

• Schulprojekt = school project

In German compounds, the last part usually determines the grammatical gender. Since das Projekt is neuter, das Schulprojekt is also neuter.

Yes, grammatically you can say welches, because welches is also a neuter singular relative pronoun.

So this is possible:

• Das Schulprojekt, welches wir im Unterricht planen, ist interessant.

But in everyday German, das is much more common and sounds more natural.

So learners should usually prefer:

• das wir im Unterricht planen

Yes. Both verbs are in the present tense:

• planen = we plan / we are planning
• ist = is

So the sentence describes something current:

• the project is currently being planned in class
• and it is interesting

As in English, the German present tense can often cover both a simple present and a present progressive meaning, depending on context.

Sometimes yes, but planen must stay at the end of the relative clause.

Current version:

• das wir im Unterricht planen

You may also hear:

• das wir planen
• das wir gerade im Unterricht planen

The important rule is that in the relative clause, the finite verb planen goes to the end.

So this would be wrong:

• das wir planen im Unterricht ❌

It may be understandable, but it does not follow normal German subordinate-clause word order.

Without the relative clause, the sentence is simply:

• Das Schulprojekt ist interessant.

The relative clause just adds extra information:

• Das Schulprojekt, das wir im Unterricht planen, ist interessant.

This is a useful way to analyze long German sentences:

1. Find the main clause.
2. Identify any inserted relative clause.
3. Read the main clause again without the insertion.

That often makes the structure much easier to understand.