Ich dachte, der Termin sei morgen, aber ich habe mich geirrt.

Termin usually means a scheduled appointment, meeting, or set date/time for something.

In this sentence, der Termin is best understood as the appointment or the scheduled date.

A common learner mistake is to connect Termin with English term, but they are usually not the same. German Termin does not normally mean a school term or an abstract term/word.

Dachte is the simple past tense of denken.

German often uses the simple past (Präteritum) with common verbs like sein, haben, wissen, denken, especially in writing, narration, and often even in speech.

So:

• Ich dachte ... = very natural
• Ich habe gedacht ... = possible, but often sounds a bit less neutral here

In this sentence, Ich dachte is the most idiomatic choice.

Sei is a form of sein in the Konjunktiv I.

Here it is used to show reported thought or indirect statement:

• Ich dachte, der Termin sei morgen.

This means something like:

• I thought the appointment was tomorrow
• more literally: I believed that the appointment was tomorrow

The Konjunktiv I adds a slight sense of distance: it presents the content as what someone thought or said, not as a fact the speaker is confirming.

Because the sentence is presenting what the speaker believed at the time, not what is actually true.

Using sei marks the clause as dependent on the thought:

• Ich dachte, der Termin sei morgen.

This is a fairly elegant or formal way to express that idea.

In everyday German, many speakers would also simply say:

• Ich dachte, der Termin ist morgen.

That is very common and natural too.
So the difference is mostly one of style and grammatical explicitness:

• sei = more formal, more clearly marks indirect thought/reporting
• ist = very common in everyday speech

Yes. In everyday spoken German, that is extremely common.

So both are possible:

• Ich dachte, der Termin sei morgen.
• Ich dachte, der Termin ist morgen.

The version with sei sounds more formal, more careful, or more literary.
The version with ist sounds more conversational.

Because sei is the normal Konjunktiv I form of sein, and that is the usual choice for indirect speech or indirect thought.

• sei = Konjunktiv I
• wäre = Konjunktiv II

Wäre usually suggests something more hypothetical, less direct, or more removed from reality.

So in this sentence, sei is the better and more standard choice if you want an indirect-thought reading.

German often leaves out dass after verbs like denken, sagen, glauben, wissen, especially when the meaning is already clear.

So these are both possible:

• Ich dachte, der Termin sei morgen.
• Ich dachte, dass der Termin morgen sei.

Without dass, the sentence sounds a bit more compact and natural.
With dass, it can sound slightly more explicit or formal.

Because der Termin is the subject of the clause der Termin sei morgen.

If you ask What is tomorrow?, the answer is the appointment. That makes der Termin the subject, so it stays in the nominative:

• der Termin

not accusative:

• den Termin

Because the verb is sich irren, which means to be mistaken.

It is a reflexive verb, so it needs a reflexive pronoun:

• ich irre mich
• du irrst dich
• er irrt sich

In the perfect tense:

• ich habe mich geirrt

You should learn it as a unit:

• sich irren = to be mistaken

It is not natural in modern German to say ich habe geirrt with this meaning.

Both are grammatically possible, but the perfect tense is much more common in everyday spoken German for a verb like sich irren.

So:

• Ich habe mich geirrt. = the usual spoken form
• Ich irrte mich. = correct, but more literary or formal

This creates an interesting contrast in the full sentence:

• Ich dachte ... uses simple past, which is very normal with denken
• ich habe mich geirrt uses the perfect, which is very normal with sich irren

So the tense choice is idiomatic, not random.

Because here morgen means tomorrow, so it is an adverb.

Adverbs are not capitalized in German:

• Der Termin sei morgen.

But the noun der Morgen meaning the morning is capitalized:

• am Morgen = in the morning

So:

• morgen = tomorrow
• Morgen = morning / tomorrow when used as a noun in certain contexts

There are two clauses joined together, and German uses commas to separate them.

1. Ich dachte, der Termin sei morgen

   • The comma separates the main clause from the clause expressing the content of the thought.

2. ..., aber ich habe mich geirrt

   • Aber joins another clause, and a comma is placed before it.

So the commas help show the structure clearly:

• Ich dachte, / der Termin sei morgen, / aber ich habe mich geirrt.