Learn Hardcore German: Ich habe lange über meinen Plan nachgedacht, und meine Schwester hat mich dazu ermutigt. - I thought about my plan for a long time, and my sister encouraged me to do it.

haben with most verbs
sein mainly with:
- verbs of motion/change of place (gehen, kommen, fahren …)
- verbs of change of state (einschlafen, sterben …)
- a few special intransitive verbs (bleiben, sein, passieren …)

nachdenken (“to reflect / to think something over”) describes a mental activity, not motion or change of state, so it uses haben:

✅ Ich habe über meinen Plan nachgedacht.
❌ Ich bin über meinen Plan nachgedacht. (wrong)

What is the difference between denken and nachdenken, and why is it nachgedacht here?

denken = “to think” in a general sense
- Ich denke oft an dich. – I often think of you.
nachdenken (über + Akk.) = “to think about something carefully, to reflect on it”
- Ich muss über dieses Problem nachdenken. – I need to think this problem through.

In your sentence, the idea is “I thought about my plan for a long time / I reflected on my plan”, so nachdenken (über) is the natural choice. Its past participle is:

denken → gedacht
nachdenken → nachgedacht (prefix nach- goes in front of ge-)

Why is it über meinen Plan and not something like an meinen Plan?

With verbs of speaking, thinking, or reflecting, German usually uses:

über + Akkusativ = about / concerning
- nachdenken über etwas – think about something
- sprechen über etwas – talk about something

an + Akkusativ with denken is also possible but has a different nuance:

an etwas denken = have something in mind, remember something
- Ich denke an dich. – I’m thinking of you / you’re on my mind.

Here the idea is “reflecting on / considering a plan”, so:

✅ über meinen Plan nachdenken (reflect on my plan)
❌ an meinen Plan nachdenken (unidiomatic)

Why is it meinen Plan and not meinem Plan?

The form changes because of case:

The preposition über in the sense of “about, concerning” takes the accusative.
Plan is masculine: der Plan.
Masculine accusative of mein is meinen.

So:

Nominative: mein Plan (my plan – subject)
Accusative: meinen Plan (my plan – object after über)

Since über here requires accusative, we get über meinen Plan.

Why is lange placed before über meinen Plan, and could the order be different?

In Ich habe lange über meinen Plan nachgedacht:

lange is an adverb (“for a long time”),
über meinen Plan is a prepositional phrase (“about my plan”),
nachgedacht is the main verb at the end (because of the perfect tense and separable verb).

A natural order is:

Time adverbials
Other information
Past participle at the end

So lange comes before über meinen Plan.

You can say:

Ich habe über meinen Plan lange nachgedacht.

This is grammatically correct but a bit less neutral; it slightly emphasizes Über meinen Plan first. The original word order is more typical and smoother in everyday speech.

Why does nachdenken split, and how does that work in different tenses?

nachdenken is a separable verb (trennbares Verb):

Prefix: nach-
Verb stem: denken

Rules:

Present / simple past in a main clause
Prefix goes to the end:
- Ich denke über meinen Plan nach.
- Ich dachte über meinen Plan nach.
Perfect tense
The participle is nachgedacht (nach-
- ge-
  - dacht), and it goes to the end:
- Ich habe über meinen Plan nachgedacht.
Infinitive with zu
zu goes between prefix and stem:
- Ich versuche, über meinen Plan nachzudenken.

So the splitting is a regular feature of separable verbs.

What exactly does lange mean here, and is there a difference between lang and lange?

In your sentence, lange is an adverb meaning “for a long time”:

Ich habe lange über meinen Plan nachgedacht.
→ I thought about my plan for a long time.

Some contrasts:

lang as an adjective:
- ein langer Weg – a long way
lang after sein/bleiben (predicative):
- Der Weg ist lang. – The way is long.
lange as an adverb (duration):
- Ich habe lange gewartet. – I waited for a long time.

So here you need the adverb form lange because it modifies the verb (how long you were thinking).

Why is there a comma before und, and is it mandatory?

You have two independent main clauses:

Ich habe lange über meinen Plan nachgedacht,
und meine Schwester hat mich dazu ermutigt.

In modern German:

With und joining two main clauses, the comma is optional.
Both are correct:
- Ich habe lange über meinen Plan nachgedacht(,) und meine Schwester hat mich dazu ermutigt.

Writers often use the comma if the clauses are longer or if they want to make the separation clearer, as in this sentence.

Why is the word order meine Schwester hat mich dazu ermutigt and not hat dazu mich ermutigt?

In a main clause, the finite verb (here: hat) must be in second position:

[1: meine Schwester] [2: hat] [Mittelfeld: mich dazu] [Ende: ermutigt]

In the Mittelfeld (the part between the conjugated verb and the final verb), standard word order prefers:

Pronouns (accusative/dative)
Adverbs / other elements
Then the non-finite verb at the end

So:

mich (personal pronoun, accusative) comes before
dazu (pronominal adverb)

Hence:

✅ meine Schwester hat mich dazu ermutigt
❌ meine Schwester hat dazu mich ermutigt (understandable, but unusual/marked)

What does dazu refer to here, and how do these “da-” words work?

dazu is a pronominal adverb: da- + zu.

Meaning here: “to do that / to go ahead with it / with regard to that plan.”

It refers back to the previous idea (the plan you were thinking about). In context, it often means:

She encouraged me to follow through with the plan.
She encouraged me with regard to that plan.

About pronominal adverbs:

They replace preposition + neuter pronoun (das / es).
Instead of zu dem or zu es, German uses dazu.
Common examples:
- darüber (about it)
- dafür (for it)
- damit (with it / so that)
- daran (at it / to it)

So mich dazu ermutigen = “encourage me to do that / encourage me about that.”

Why is it ermutigt and not something like ermutete?

ermutigt is the past participle of ermutigen (“to encourage”):

Infinitive: ermutigen
Stem: ermutig-
Past participle of regular verbs: ge- + stem + -t
→ ermutigt

German has two common past tenses:

Perfekt (haben/sein + Partizip II)
– Preferred in spoken and many written contexts, especially for actions in the past with present relevance.
- meine Schwester hat mich ermutigt
Präteritum (simple past)
– More written/literary for regular verbs like ermutigen:
- meine Schwester ermutigte mich

So hat mich ermutigt is the normal spoken past form; ermutete would be a different verb (ermutete would come from a hypothetical verb ermut(en) and isn’t used here).

Could I also say Ich dachte lange über meinen Plan nach, and what is the difference?

Yes, that sentence is grammatically correct:

Ich habe lange über meinen Plan nachgedacht. (Perfekt)
Ich dachte lange über meinen Plan nach. (Präteritum)

Difference:

In spoken German, the Perfekt is much more common for past events, especially with regular and many irregular verbs.
The Präteritum is very common in:
- written narratives,
- with a few frequent verbs (war, hatte, konnte, musste, wollte, ging, kam, etc.) even in speech.

So your original sentence with habe … nachgedacht sounds more natural in everyday conversation; dachte … nach feels a bit more written/narrative or formal.

Can I say Ich habe lange über meinen Plan gedacht, or is that wrong?

That sounds unnatural in German.

For “to think about something” in the sense of reflect / consider, you normally use:

über etwas nachdenken
- Ich habe lange über meinen Plan nachgedacht.

denken über etwas exists but is used more for:

having an opinion about something:
- Was denkst du über meinen Plan? – What do you think of my plan?

So:

Ich habe lange über meinen Plan nachgedacht. ✅ (I reflected on my plan)
Ich habe lange über meinen Plan gedacht. ❌ (grammatically possible but not idiomatic; a native wouldn’t say it this way in this meaning)