Ich mache den Zettel an der Pinnwand fest.

Because the verb here is festmachen, which is a separable verb.

In a normal main clause, the conjugated part goes to position 2, and the prefix goes to the end:

• Ich mache den Zettel an der Pinnwand fest.

This is very common in German:

• Ich rufe dich später an.
• Er steht früh auf.

In infinitives and past participles, the verb stays together:

• den Zettel an der Pinnwand festmachen
• Ich habe den Zettel an der Pinnwand festgemacht.

Here, festmachen means to fasten, to attach, or to secure.

The basic idea of fest is firmly / fixed / secure, so festmachen is literally something like make fast.

In this sentence, it means attaching the note so that it stays in place on the pinboard.

Depending on context, festmachen can also have other meanings, so learners often need to rely on the situation. But in this sentence, it clearly means attach securely.

Because den Zettel is the accusative case, and it is the direct object of the verb.

You are doing something to the note, so the note is the thing affected by the action.

• Nominative: der Zettel
• Accusative: den Zettel

Since Zettel is masculine, the article changes from der to den in the accusative.

Compare:

• Der Zettel ist klein.
  Here der Zettel is the subject.

• Ich mache den Zettel fest.
  Here den Zettel is the object.

This is a very common question because an is a two-way preposition and can take either:

• accusative for direction / destination
• dative for location

In this sentence, German uses an der Pinnwand with the dative because the expression etwas an etwas festmachen normally takes an + dative.

So you should learn this as a common pattern:

• etwas an der Wand festmachen
• etwas am Brett festmachen
• etwas an der Pinnwand festmachen

Even though English may feel like onto the pinboard, German usually treats this as fastening something at / on that surface, not emphasizing movement toward it.

Because an is typically used for something attached to a vertical surface or side surface.

A Pinnwand is like a board or noticeboard, and the note is fixed on it / against it, not lying flat on top of it.

So German prefers:

• an der Pinnwand
• an der Wand

By contrast, auf is more for something resting on a horizontal surface:

• Das Buch liegt auf dem Tisch.

So the difference is roughly:

• an = attached to / on the side of a surface
• auf = on top of a surface

A Pinnwand is a pinboard, noticeboard, or bulletin board.

It comes from:

• Pinn- from pinnen or the idea of pins
• Wand = wall

So it is literally a board where you pin things up.

Depending on context, translations can include:

• pinboard
• noticeboard
• bulletin board

No. German has several verbs that could fit similar situations, but they are not always exactly the same.

For example:

• befestigen = to attach / fasten
  
  • Ich befestige den Zettel an der Pinnwand.
• anheften = to pin / tack on
  
  • Ich hefte den Zettel an die Pinnwand.
• ankleben = to stick on with glue or tape
  
  • Ich klebe den Zettel an die Pinnwand.

So festmachen is a good general verb for making something secure, but the best choice depends on how you attach it.

The sentence follows normal German main-clause word order:

• Ich = subject
• mache = conjugated verb in second position
• den Zettel = object
• an der Pinnwand = prepositional phrase
• fest = separable prefix at the end

So the structure is:

Subject + finite verb + object + other information + separable prefix

This is why you get:

• Ich mache den Zettel an der Pinnwand fest.

and not:

• Ich festmache den Zettel ... in a normal main clause

In the perfect tense, the separable verb becomes a past participle with ge inserted:

• festmachen → festgemacht

So the sentence becomes:

• Ich habe den Zettel an der Pinnwand festgemacht.

That is a very useful pattern to remember:

• anmachen → angemacht
• aufmachen → aufgemacht
• festmachen → festgemacht

Yes. fest is a very common word in German and often means things like:

• firm
• secure
• fixed
• tight
• sometimes even solid

Examples:

• Die Schraube sitzt fest. = The screw is tight / stuck firmly.
• Halte es fest. = Hold it tight.
• Es steht fest. = It is certain.

So in festmachen, the idea is that you make something fixed securely.

Yes, that is a very natural English way to express it.

Depending on context, possible English translations include:

• I fasten the note to the pinboard.
• I attach the note to the pinboard.
• I pin the note up on the pinboard.
• I secure the note to the pinboard.

The exact translation depends on how literal or natural you want to sound in English.

Not exactly.

• Zettel usually means a note, slip of paper, or small written paper
• Blatt means a sheet or leaf, often a sheet of paper

So:

• ein Zettel = a note / slip of paper
• ein Blatt Papier = a sheet of paper

In this sentence, Zettel suggests a note, reminder, or small piece of paper being attached to the pinboard.