Learn Hardcore German: Mein Tag war heute erfolgreich.

Both are correct, but they don’t mean exactly the same. The original sentence is more about how the day went overall, not just about you as a person being successful.

Why Mein Tag and not Meine Tag?

Because Tag is masculine in German:

der Tag (masculine, nominative singular)

The possessive mein- has to match the noun’s gender, number, and case:

Masculine nominative singular: mein Tag
Feminine nominative singular: meine Zeit
Neuter nominative singular: mein Haus
Plural (all genders) nominative: meine Tage

In the sentence, Mein Tag is the subject (nominative), masculine singular, so you use mein, not meine.

Why is there no ending on erfolgreich here? Shouldn’t it be erfolgreicher or erfolgreichen?

Erfolgreich here is a predicate adjective, not an adjective before a noun.

Predicate adjective: comes after a form of sein, werden, bleiben, etc.
→ It does not take an ending:
- Mein Tag war erfolgreich.
- Das Ergebnis ist gut.
- Die Prüfung war schwer.
Attributive adjective: stands directly before a noun.
→ It takes an ending:
- ein erfolgreicher Tag
- mein erfolgreicher Tag
- der erfolgreiche Tag

So:

Mein Tag war heute erfolgreich. ✅ (predicate adjective, no ending)
Mein erfolgreicher Tag war heute. ✅ (attributive adjective, with ending -er)

Can I change the word order to Mein Tag war erfolgreich heute?

You can, and it’s grammatically possible, but it sounds unusual and slightly awkward in standard German.

The most natural options are:

Mein Tag war heute erfolgreich. (neutral, very natural)
Heute war mein Tag erfolgreich. (emphasizes today)

Putting heute at the very end (… erfolgreich heute) is typically used only for special emphasis or in poetic/creative language. In everyday speech/writing, keep heute before the adjective.

Why is heute in the middle of the sentence and not at the beginning?

Both are correct; they just sound a bit different:

Mein Tag war heute erfolgreich.
→ Neutral; focus on “my day” and what it was like.
Heute war mein Tag erfolgreich.
→ Emphasis on today (as opposed to other days).

German main clauses often follow this pattern:

(Optional element) – Verb – Subject – (other parts)

So if you start with heute, you must move the verb to second position:

Heute war mein Tag erfolgreich. ✅
Heute mein Tag war erfolgreich. ❌ (verb must come second)

Why is the verb war and not ist?

War is the simple past (Präteritum) of sein:

ich war – I was
er/sie/es war – he/she/it was

Using war shows that you are looking back on (most of) the day as something completed:

Mein Tag war heute erfolgreich.
= You are reflecting on the day that (almost) has passed.

Mein Tag ist heute erfolgreich would normally be used while the day is still ongoing and you’re describing the current state. In practice, Germans often still use war even if the day isn’t completely over yet, because they’re talking about how it has gone so far.

Could I say Mein Tag ist heute erfolgreich gewesen instead of war?

Yes, it is grammatically correct, but it sounds somewhat heavy or overly formal in everyday speech.

Mein Tag war heute erfolgreich.
→ Normal, natural, especially in writing but also acceptable in speech.
Mein Tag ist heute erfolgreich gewesen.
→ Perfect tense (Perfekt). Grammatically fine, but most speakers would prefer the shorter war here. The perfect with gewesen is often used when you want to emphasize completion or in specific contexts, but with adjectives like erfolgreich it can feel stiff.

Spoken German usually uses Perfekt for many verbs, but sein and haben in simple statements like this are very commonly used in Präteritum:
Ich war müde. Es war schön. Mein Tag war erfolgreich.

What’s the difference between Mein Tag war heute erfolgreich and Ich hatte heute einen erfolgreichen Tag?

Both are correct and very close in meaning, but the structure and nuance differ:

Mein Tag war heute erfolgreich.
- Subject: Mein Tag
- Verb: war (sein)
- Adjective: erfolgreich (predicate)
- Feels slightly more abstract: “The day as a whole was successful.”
Ich hatte heute einen erfolgreichen Tag.
- Subject: Ich
- Verb: hatte (haben, simple past)
- Object: einen erfolgreichen Tag (accusative; attributive adjective with ending -en)
- Sounds a bit more colloquial and personal: “I had a successful day today.”

Both are natural. In everyday speech, Ich hatte heute einen erfolgreichen Tag may sound a bit more common, but both are fine.

Does erfolgreich here just mean “good” or does it specifically mean “successful”?

Erfolgreich specifically means successful in the sense of:

achieving goals,
having good results,
being effective/productive.

So Mein Tag war heute erfolgreich suggests that things you planned actually worked out (e.g. you finished tasks, closed deals, passed exams).

If you just want to say the day was pleasant or nice, you’d normally use:

Mein Tag war heute gut. – My day was good.
Mein Tag war heute schön. – My day was nice/pleasant.
Mein Tag war heute entspannt. – My day was relaxed.

Erfolgreich is more about success than about mood or comfort.

Why is Tag masculine? Do I just have to memorize that?

Yes. In German, every noun has a grammatical gender (masculine, feminine, neuter), and Tag happens to be:

der Tag (masculine)

There is no reliable rule to predict the gender of Tag from its meaning; it’s something you memorize together with the noun:

der Tag – masculine
die Nacht – feminine
das Jahr – neuter

When you learn nouns, it helps to always learn them with their article:

Not just Tag, but der Tag
Then it’s easier to choose forms like mein Tag, einen Tag, des Tages, etc.