Das Whiteboard ist heute sauber, aber die Folie auf dem Bildschirm ist zu klein.

Because Whiteboard is normally a neuter noun in German, so its nominative singular article is das.

A good habit is to learn German nouns together with their article:

• das Whiteboard
• die Folie
• der Bildschirm

German noun gender often does not match anything logical in English, so it usually has to be memorized.

It is a very common borrowed word from English, and German speakers do use Whiteboard in everyday speech, especially in schools and offices.

German often keeps borrowed nouns like this, but still gives them a grammatical gender and treats them like normal German nouns. So even though the word looks English, the grammar around it is German: das Whiteboard, dem Whiteboard, and so on.

In a normal German main clause, the finite verb goes in the second position. Here:

• Das Whiteboard = first position
• ist = second position
• heute sauber = the rest of the clause

So heute comes after the verb because ist must stay in second position.

You could also say:

• Heute ist das Whiteboard sauber

That is also correct, but it gives heute more emphasis.

Because aber joins two full main clauses, and each clause needs its own finite verb.

So you have:

• Das Whiteboard ist heute sauber
• aber die Folie auf dem Bildschirm ist zu klein

German does not let the second clause borrow the verb from the first one here. Each clause stands on its own grammatically.

No. Aber is a coordinating conjunction, so the clause after it keeps normal main-clause word order.

That is why you get:

• ..., aber die Folie auf dem Bildschirm ist zu klein.

The verb ist is still in second position within that clause.

This is different from subordinating conjunctions like weil, which send the verb to the end:

• ..., weil die Folie auf dem Bildschirm zu klein ist.

Here, die Folie means a slide, especially a presentation slide being shown on a screen.

This word can have other meanings in other contexts, such as:

• plastic film
• foil
• transparency

But in a sentence with auf dem Bildschirm, the most natural meaning is slide.

Because Folie is a feminine noun in German, so its nominative singular article is die.

In this sentence, die Folie is the subject of the second clause:

• die Folie ... ist zu klein

Since it is the subject, it is in the nominative case, and the nominative singular feminine article is die.

Because auf can take either the dative or the accusative, depending on the meaning.

• dative = location, where something is
• accusative = direction or movement toward something

Here the slide is already located on the screen, so German uses the dative:

• auf dem Bildschirm

Since Bildschirm is masculine, the dative singular article is dem.

Compare:

• Die Folie ist auf dem Bildschirm. = The slide is on the screen.
• Ich bringe die Folie auf den Bildschirm. = I bring the slide onto the screen.

Zu before an adjective means too.

So:

• klein = small
• zu klein = too small

That means the slide is not just small, but smaller than is acceptable or useful.

Because after sein, these are predicate adjectives, and predicate adjectives do not take adjective endings in German.

So you say:

• Das Whiteboard ist sauber
• Die Folie ist zu klein

Adjective endings are used when the adjective comes directly before a noun:

• ein sauberes Whiteboard
• eine kleine Folie

But after ist, you use the basic form:

• sauber
• klein

Because all nouns are capitalized in German, not just proper names.

So in this sentence:

• Whiteboard is a noun
• Folie is a noun
• Bildschirm is a noun

That is why they all begin with a capital letter. This is one of the most noticeable spelling rules in German.