Wir notieren den Abgabetermin fett im Kalender.

Because Abgabetermin is the direct object of notieren.

The verb notieren usually takes an accusative object, so:

• der Abgabetermin = nominative form
• den Abgabetermin = accusative form

You can see the same pattern with other masculine nouns:

• Der Lehrer kommt.
• Ich sehe den Lehrer.

Since Abgabetermin is masculine (der Abgabetermin), its article changes to den in the accusative.

Abgabetermin is a compound noun:

• Abgabe = submission, handing in
• Termin = appointment, date, scheduled time

So literally it is something like submission date or deadline for handing something in.

In German, compound nouns are very common, and the last part determines the gender. Since Termin is masculine (der Termin), the whole word is also masculine:

• der Abgabetermin

Because here fett is not being used as an adjective before a noun. It is being used adverbially to describe how the deadline is noted.

So:

• ein fetter Text = a bold/fat text
  Here fetter is an adjective and changes form.
• Wir notieren es fett. = We write/note it in bold.
  Here fett functions like an adverb, so it does not change.

This is very common in German: the same word can be adjective or adverb, and in adverb use it usually stays in the basic form.

Not in this sentence.

fett can mean fat or greasy in some contexts, but with writing or formatting it means bold.

For example:

• fette Schrift = bold type / bold font
• etwas fett schreiben = to write something in bold
• fett markieren = to highlight/emphasize in bold

So in this sentence, fett means in bold.

Because im Kalender means in the calendar as a location, and that uses the dative.

• im = contraction of in dem
• dem is dative singular

German often uses:

• in + dative for location
• in + accusative for movement/direction

So:

• im Kalender = in the calendar
• in den Kalender = into the calendar

In real usage, both can make sense depending on what you want to emphasize:

• Wir notieren den Abgabetermin im Kalender.
  We note it in the calendar.
• Wir tragen den Abgabetermin in den Kalender ein.
  We enter the deadline into the calendar.

The given sentence uses im Kalender, which focuses on where it appears.

im is a contraction of in dem.

German often combines certain prepositions and articles:

• in dem → im
• an dem → am
• zu dem → zum
• zu der → zur

So:

• im Kalender = in dem Kalender

This is standard and very common.

German main clauses normally put the finite verb in second position:

• Wir = first element
• notieren = second element

After that, German is more flexible, but a very natural order is:

1. subject
2. verb
3. object
4. manner
5. place

So here:

• Wir = subject
• notieren = verb
• den Abgabetermin = object
• fett = manner/how
• im Kalender = place/where

That gives:

Wir notieren den Abgabetermin fett im Kalender.

You could sometimes rearrange parts for emphasis, but the given order is normal and clear.

Yes, that is grammatically possible, but it sounds less neutral.

German allows some flexibility in word order, especially after the verb, but different orders shift the emphasis. The original version sounds more straightforward:

Wir notieren den Abgabetermin fett im Kalender.

If you say:

Wir notieren im Kalender den Abgabetermin fett,

it puts more focus on im Kalender early in the sentence. That may be useful in a specific context, but it is less neutral as a default sentence.

Not exactly.

• schreiben = to write
• notieren = to note down, jot down, record

notieren often suggests writing something down as information to remember or keep track of.

So in this sentence, notieren sounds a bit more purposeful than just schreiben.

Compare:

• Ich schreibe die Nummer auf. = I write down the number.
• Ich notiere die Nummer. = I note down the number.

Both can work, but notieren often feels a bit more formal or organized.

They are capitalized for different reasons:

• Wir is capitalized because it is the first word of the sentence.
• Kalender is capitalized because it is a noun.

In German, all nouns are capitalized, not just proper names.

So in this sentence, the nouns are:

• Abgabetermin
• Kalender

Both must be capitalized.

Yes, and in many contexts eintragen would sound very natural.

• notieren = note down
• eintragen = enter, write into a list/calendar/form

For a calendar, German often uses eintragen:

• Wir tragen den Abgabetermin fett in den Kalender ein.

That sounds like We enter the deadline in bold into the calendar.

The sentence with notieren is still understandable and correct, but eintragen is especially common when you add something to a calendar, list, or form.

Not really as a strict fixed expression, but it is a very natural combination.

German often uses formatting words like this with verbs of writing or marking:

• etwas fett schreiben = write something in bold
• etwas fett markieren = mark something in bold
• etwas fett drucken = print something in bold
• etwas fett notieren = note something in bold

So it is understandable and idiomatic, even if fett schreiben may be the most common wording in some contexts.