Learn Hardcore German: Am Abend denke ich über meinen Tag nach.

Both are correct. Putting Am Abend first simply emphasizes the time (“In the evening, I think about my day”) and sounds very natural in German. The verb denke still stays in second position; Am Abend counts as the first sentence element.

What exactly is am in Am Abend?

Am is a contraction of an + dem:

an dem Abend → am Abend

Here:

an is a preposition.
dem is the dative form of the masculine article der.
Abend is masculine (der Abend).

For time expressions, an + dative is common:

am Morgen (in the morning)
am Nachmittag (in the afternoon)
am Abend (in the evening)

What case is Abend in Am Abend, and why?

Abend is in the dative singular:

am = an dem → dem is dative.
So the full underlying phrase is an dem Abend.

With time expressions like am Abend, German uses the dative after an to mean “on/in the evening” (a point in time).

What’s the difference between am Abend and abends?

Both relate to “in the evening,” but:

am Abend = on that (specific) evening or in a more concrete sense.
- Am Abend lese ich ein Buch.
  → This evening / in the evening (as a particular part of the day), I read a book.
abends = in the evenings / in the evening, generally, as a habit.
- Abends lese ich ein Buch.
  → I read a book in the evenings (as a usual routine).

So your sentence with Am Abend can describe a specific time of day, and with Abends it would highlight a regular habit:

Abends denke ich über meinen Tag nach.
→ In the evenings, I (usually) reflect on my day.

Why are denke and nach separated in denke ich … nach?

Because nachdenken is a separable verb.

The base form is nachdenken (“to reflect / to think about (in a reflective way)”).
In main clauses in the present or simple past, separable verbs split:
- The stem (here: denke) goes in second position.
- The prefix (here: nach) goes to the very end of the clause.

So:

Ich denke über meinen Tag nach.
Am Abend denke ich über meinen Tag nach.

In the infinitive or with a modal verb, you see it together again:

Ich will über meinen Tag nachdenken.
Über meinen Tag nachzudenken ist wichtig für mich.

Where does nach go in different kinds of sentences with nachdenken?

General rule for separable verbs like nachdenken:

Main clause, finite verb in 2nd position
Prefix at the end:
- Ich denke über meinen Tag nach.
- Morgen denke ich darüber nach.
Subordinate clause (introduced by weil, dass, wenn, etc.)
Verb (with prefix) goes to the end, not split:
- …, weil ich über meinen Tag nachdenke.
- …, dass ich oft über meinen Tag nachdenke.
Infinitive with zu
Prefix + stem stay together:
- über meinen Tag nachzudenken
- Es ist gut, über seinen Tag nachzudenken.
With a modal verb (können, wollen, müssen, etc.)
Infinitive at the end; prefix + stem together:
- Ich will über meinen Tag nachdenken.

How is nachdenken different from simply denken?

denken (an/über …) = to think (of / about).
More general: to have someone/something in your thoughts.
nachdenken (über …) = to reflect, to think something over, to consider carefully.

Examples:

Ich denke an dich.
→ I’m thinking of you (you’re on my mind).
Ich denke über das Problem nach.
→ I’m reflecting on the problem / thinking it over.

In your sentence, über meinen Tag nachdenken means you are reflecting on your day, not just briefly having it in mind.

Why is über used here? Could you use an instead?

Both über and an can appear with denken, but they’re used differently:

an + accusative with denken:
- an etwas / jemanden denken = to think of something/someone (they come to mind).
- Ich denke an meinen Tag. → I think of my day (it crosses my mind).
über + accusative with (nach)denken:
- über etwas (nach)denken = to think about something more deeply or reflect on it.
- Ich denke über meinen Tag nach. → I reflect on my day.

In your sentence, über is natural because the meaning is reflective: you’re going through your day in your mind, not just briefly recalling it.

Why is it über meinen Tag and not über mein Tag?

Because Tag is:

masculine: der Tag
here it’s the object of the preposition über with the meaning “about,” which always takes the accusative.

So you need accusative masculine:

Nominative: der Tag, mein Tag
Accusative: den Tag, meinen Tag

Therefore:

über meinen Tag = about my day (correct)
über mein Tag = ungrammatical here

Could it be über meinem Tag with -em instead of -en?

No, not in this meaning.

Explanation:

über is a two-way preposition (Wechselpräposition). It can take dative or accusative:
- Usually dative for location (where?).
- Accusative for direction/motion (where to?) or for abstract meanings like “about.”

In the sense of “about” (thinking/talking about something), über always uses the accusative:

Ich spreche über meinen Tag.
Ich denke über meinen Tag nach.

Meinem Tag would be dative masculine, which would fit only in a different kind of phrase, for example:

über meinem Tag in a literal spatial sense (something is located above my day) — which doesn’t make sense here.

What would the form be if the noun weren’t masculine, but feminine or neuter?

Using the same pattern über meinen Tag, but changing the noun:

Masculine: der Tag
- Accusative: meinen Tag
- Ich denke über meinen Tag nach.
Feminine: die Woche
- Accusative: meine Woche
- Ich denke über meine Woche nach.
Neuter: das Jahr
- Accusative: mein Jahr
- Ich denke über mein Jahr nach.

So only the masculine form adds -en in the accusative; feminine and neuter look the same as the nominative with mein.

Can I change the word order, for example: Ich denke am Abend über meinen Tag nach? Is that also correct?

Yes, that is correct:

Am Abend denke ich über meinen Tag nach.
Ich denke am Abend über meinen Tag nach.
Ich denke über meinen Tag am Abend nach.

All three are grammatical. The core rules you must keep:

Finite verb second in a main clause:
- denke must be in position 2.
Prefix of a separable verb at the end:
- nach must be at the very end of the clause.

So something like:

✗ Am Abend denke ich nach über meinen Tag.

is wrong, because nach must come last.

What tense is denke here, and could you use a different tense?

Denke is present tense (ich denke).

German present tense is very flexible. It can express:

An action happening now.
A regular habit.
A future action, especially when a time expression is present.

Your sentence can mean:

“In the evening, I (typically) think about my day.” (habit)
“This evening I’ll think about my day.” (future, from context)

Other tenses:

Simple past:
Am Abend dachte ich über meinen Tag nach.
→ In the evening, I thought about my day.
Present perfect (very common in spoken German):
Am Abend habe ich über meinen Tag nachgedacht.
→ In the evening, I have thought / I thought about my day.