Learn Hardcore German: Der Termin passt gut in meinen Kalender. - The appointment fits well into my calendar.

In this sentence, Der Termin is the subject, so it must be in the nominative case.
Masculine, nominative singular → the definite article is der.

Other forms of Termin for comparison:

Nominative: der Termin (subject)
Accusative: den Termin (direct object)
Dative: dem Termin
Genitive: des Termins

Why is the verb passt in second position? Could the word order be different?

German main clauses follow the verb-second rule (V2):

The finite verb (here: passt) must be the second element in the sentence.

In Der Termin passt gut in meinen Kalender.:

First element: Der Termin (one noun phrase)
Second element: passt (the conjugated verb)
Everything else follows: gut in meinen Kalender

You can move another element to the front, but passt stays second:

In meinen Kalender passt der Termin gut.
Gut passt der Termin in meinen Kalender.

If you move passt away from second position (without making a question or a subordinate clause), it becomes incorrect standard German.

What exactly does passen mean here, and how is it used in German?

In this sentence, passen means something like:

“to fit / to work / to be convenient.”

Common patterns with passen:

passen + jemandem (Dativ) – “to suit someone / to be convenient for someone”
- Der Termin passt mir gut. – The appointment suits me / works well for me.
passen in + Akkusativ – “to fit (into something)”
- Der Termin passt gut in meinen Kalender. – The appointment fits well into my schedule.
passen zu + Dativ – “to match / go with”
- Die Schuhe passen gut zu der Hose. – The shoes go well with the pants.

So here, passen in is used to say the time fits well into the available space in your calendar.

Why is it in meinen Kalender and not in meinem Kalender?

The preposition in can take either accusative or dative, depending on the meaning:

in + accusative → movement/direction (into some space)
in + dative → location/state (in some space, no movement)

In this sentence:

in meinen Kalender → accusative
It suggests putting/entering the appointment into the calendar, i.e. it fits into your schedule.

If you said in meinem Kalender (dative), it would describe something that is already in the calendar:

Der Termin steht schon in meinem Kalender.
The appointment is already in my calendar.

So:

passt gut in meinen Kalender = it can be entered without problems; it fits into your schedule.
ist in meinem Kalender = it is already entered there.

Why do we say meinen Kalender and not just mein Kalender?

Kalender is masculine: der Kalender.
After the preposition in (with direction), we need the accusative case:

Masculine accusative of mein is meinen.

So:

Nominative: mein Kalender – my calendar (as subject)
- Mein Kalender ist voll. – My calendar is full.
Accusative: meinen Kalender – my calendar (as object)
- Der Termin passt in meinen Kalender.

The -en ending on meinen shows masculine accusative.

What does gut do in this sentence, and can it go somewhere else?

Here gut is an adverb, describing how well the appointment fits:

Der Termin passt gut in meinen Kalender.
→ The appointment fits my calendar well.

Typical, neutral word order is: subject – verb – (adverb) – rest.

Other positions are possible but less neutral and change the emphasis, not the basic meaning:

Der Termin passt in meinen Kalender gut. (emphasis on gut)
Gut passt der Termin in meinen Kalender. (strong emphasis, more stylistic)

For everyday speech, passt gut in meinen Kalender is the natural choice.

Could I also say Der Termin passt mir gut or Der Termin passt gut für mich? What’s the difference?

Yes, you can say:

Der Termin passt mir gut.
→ The appointment suits me well / That time works well for me.

Difference in focus:

Der Termin passt gut in meinen Kalender.
Focus: it fits into your schedule/calendar (you have a free slot).
Der Termin passt mir gut.
Focus: it suits you personally (you like that time, it’s convenient).

About für mich:

Der Termin passt gut für mich.
This is heard, but many native speakers find passen + Dativ (passt mir gut) more idiomatic.
passen für sounds a bit more casual/colloquial or influenced by English “works for me”.

Is Termin exactly the same as English “appointment”? When else is it used?

Termin often corresponds to “appointment”, but its use is a bit broader:

Official / fixed appointments:
- Arzttermin – doctor’s appointment
- Behördentermin – appointment at a government office
- einen Termin machen / vereinbaren – to make an appointment
Scheduled meeting time (business, school, etc.):
- Wir haben einen Termin um 15 Uhr. – We have a meeting/slot at 3 pm.

Close but different words:

die Verabredung – usually a private, social arrangement (a date, meeting with friends)
das Treffen – a meeting/gathering (focus on people meeting, not on the time slot)
die Frist – a deadline (not a meeting time)
das Datum – a calendar date (e.g. 5. März), not an appointment itself

Does Kalender here mean a physical calendar book, or can it also mean my digital schedule?

Kalender can refer to:

a paper diary/planner
a wall calendar
a digital calendar (e.g. on your phone or in Outlook)

In this sentence, it’s used in the same abstract way as in English “my calendar” / “my schedule”:

Der Termin passt gut in meinen Kalender.
→ It fits well into the free time slots you have, regardless of whether that calendar is physical or digital.

How would this sentence look in other tenses?

Same structure, different verb form of passen:

Present (now):
Der Termin passt gut in meinen Kalender.
The appointment fits / works well in my calendar.
Simple past (written, more formal):
Der Termin passte gut in meinen Kalender.
The appointment fit / worked well in my calendar.
Present perfect (common in speech for past events):
Der Termin hat gut in meinen Kalender gepasst.
The appointment fit / worked well in my calendar.
Future:
Der Termin wird gut in meinen Kalender passen.
The appointment will fit / work well in my calendar.