Mein Partner kommt morgen.

Because Partner is masculine singular here, and it is the subject of the sentence, so it is in the nominative case.

The possessive word mein changes like an ein-word:

• mein Partner = my partner (masculine, nominative)
• meine Partnerin = my partner (feminine, nominative)
• mein Kind = my child (neuter, nominative)
• meine Freunde = my friends (plural, nominative)

So mein Partner is the correct form for a male or grammatically masculine Partner.

Partner can mean a romantic partner, but in some contexts it can also mean a business partner or someone you work with.

Without extra context, many people will understand mein Partner as my partner in the romantic sense. If you want to be more specific, German also has:

• mein Freund = my boyfriend / male friend
• meine Freundin = my girlfriend / female friend
• mein Ehemann / meine Ehefrau = my husband / wife
• mein Geschäftspartner = my business partner

So Partner is fairly natural, but context matters.

Because all nouns are capitalized in German.

So in this sentence:

• mein = not capitalized, because it is a possessive determiner
• Partner = capitalized, because it is a noun
• kommt = not capitalized, because it is a verb
• morgen = not capitalized here, because it is being used as an adverb

This is one of the most noticeable spelling differences between German and English.

Because the subject is mein Partner, which is third person singular.

The verb kommen means to come, and it is conjugated like this in the present tense:

• ich komme
• du kommst
• er/sie/es kommt
• wir kommen
• ihr kommt
• sie/Sie kommen

Since mein Partner works like er, you need kommt:

• Mein Partner kommt morgen.

German very often uses the present tense to talk about the future, especially when there is a clear time word like morgen.

So:

• Mein Partner kommt morgen.

naturally means My partner is coming tomorrow.

German can also use the future with werden:

• Mein Partner wird morgen kommen.

But in everyday speech, the simple present is usually more common when the time is already clear.

Here, morgen means tomorrow.

That is because morgen is being used as an adverb.

Compare:

• morgen = tomorrow
• der Morgen = the morning

Examples:

• Mein Partner kommt morgen. = My partner is coming tomorrow.
• Am Morgen kommt mein Partner. = My partner is coming in the morning.

A useful thing to notice is that the noun Morgen is capitalized, but the adverb morgen is not, unless it happens to be at the beginning of the sentence.

Because mein already does the job that an article would normally do.

In English, you also say:

• my partner
• not the my partner

German works the same way:

• mein Partner
• not der mein Partner

Words like mein, dein, sein, ihr, unser, and so on are called possessive determiners, and they usually replace the article.

Because in a normal German main clause, the finite verb usually comes in second position. This is often called the V2 rule.

In this sentence:

• Mein Partner = first element
• kommt = second element
• morgen = the rest

So:

• Mein Partner kommt morgen.

But German can move other elements to the front, and the verb still stays second:

• Morgen kommt mein Partner.

Both are correct. The second version puts more emphasis on morgen.

Yes, absolutely.

Both of these are correct:

• Mein Partner kommt morgen.
• Morgen kommt mein Partner.

The difference is mainly emphasis:

• Mein Partner kommt morgen. focuses more neutrally on the whole statement.
• Morgen kommt mein Partner. highlights morgen more strongly, as in Tomorrow, my partner is coming.

This is a very common feature of German word order: different elements can come first, but the conjugated verb still stays in second position.

You would normally use Partnerin instead of Partner, and the possessive would change too:

• Meine Partnerin kommt morgen.

That is because Partnerin is a feminine noun, so the possessive determiner becomes meine in the nominative singular.

Compare:

• mein Partner = my male partner
• meine Partnerin = my female partner

A basic negated version would be:

• Mein Partner kommt morgen nicht.

This usually means something like My partner is not coming tomorrow.

If you want to be very specific that tomorrow is what you are negating, you can say:

• Mein Partner kommt nicht morgen.

That sounds more like:

• My partner is not coming tomorrow
  (maybe on a different day instead)

So the placement of nicht can slightly change what is being emphasized or negated.