Learn Hardcore German: Du bist meine liebste Freundin.

The noun here is Freundin (friend, female), which is:
- singular
- feminine
- in the nominative case (it’s a predicate noun referring back to du)

The nominative feminine ending for mein is meine:

mein Freund – my (male) friend – masculine, nominative
meine Freundin – my (female) friend – feminine, nominative

So you need meine to match Freundin.

What case is meine liebste Freundin in this sentence?

It’s nominative.

Structure:

Du – subject (nominative)
bist – verb (sein)
meine liebste Freundin – predicate nominative (a noun phrase that renames the subject)

With the verb sein (to be), the noun/adjective phrase after the verb is usually also in the nominative:

Du bist meine Freundin. – You are my friend.
Er ist mein Bruder. – He is my brother.

So meine liebste Freundin is nominative feminine singular.

Why liebste and not liebster or liebstes?

Liebste / liebster / liebstes is the superlative of lieb (dear, beloved), and it changes its ending to match the noun:

masculine nominative: liebster Freund – dearest (male) friend
feminine nominative: liebste Freundin – dearest (female) friend
neuter nominative: liebstes Kind – dearest child

Because Freundin is feminine, you use liebste.

Is liebste here a superlative? What exactly does it mean?

Yes. Liebste is the superlative form of lieb.

Rough meanings:

lieb – dear, kind, nice
lieber / liebe – dearer / more dear
liebste – dearest, most dear

In practice, meine liebste Freundin means something like:

my dearest (female) friend
my most beloved (female) friend

It’s stronger and more emotional than just meine liebe Freundin (my dear friend).

Is meine liebste Freundin romantic, or can it be just a close friend?

It can be either, depending on context, tone, and relationship:

Romantic partner:
A man or woman might say this to a girlfriend to mean my dearest (girl)friend / my beloved girlfriend.
Very close female friend (non-romantic):
Among close friends, especially women, it can also mean my dearest (female) friend, like a very strong friendship.

German Freundin alone can mean both girlfriend (romantic) and (female) friend, so the interpretation always depends on context.

What’s the difference between Freund and Freundin?

Freund – male friend; also boyfriend, depending on context
Freundin – female friend; also girlfriend, depending on context

Examples:

Das ist mein Freund. – This is my (boy)friend / boyfriend.
Das ist meine Freundin. – This is my (girl)friend / girlfriend.

To make it clearer, people sometimes add extra words:

ein guter Freund – a good (male) friend
eine gute Freundin – a good (female) friend
mein fester Freund / meine feste Freundin – my steady boyfriend / girlfriend

What’s the difference between liebste Freundin and beste Freundin?

meine liebste Freundin – my dearest friend, very emotional, affectionate, says how precious the person is to you
meine beste Freundin – my best (female) friend, often used like “best friend” in English (can be more about closeness and status than affection style)

You can be someone’s beste Freundin without them necessarily calling you their liebste Freundin, and vice versa, depending on personality and how emotional their language is.

Why is Du not capitalized here? I’ve seen Du with a capital D in some texts.

The standard modern rule:

du is normally not capitalized in the middle of a sentence:
- Du bist meine liebste Freundin.

Historically and in letters/emails, people often wrote Du, Dich, Dein with a capital letter as a sign of respect or affection. You will still see that, especially:

in personal letters
in greeting cards
in religious texts addressing God

Both Du and du are commonly seen in such personal contexts; only Sie (formal you) is always capitalized.

Why do we say Du bist meine liebste Freundin and not Du bist liebste meine Freundin or something like that?

In German, adjectives that directly describe a noun normally come before that noun in a fixed order:

Possessive / other determiner – meine
Adjective(s) – liebste
Noun – Freundin

So the natural order is:

meine liebste Freundin – my dearest friend

Putting liebste after the noun (meine Freundin liebste) is wrong in normal German. Unlike some languages, German doesn’t usually place adjectives after the noun unless they are in a separate clause:

meine Freundin, die liebste von allen – my friend, the dearest of all

How would the sentence change if I were talking to a male friend instead?

You only need to change the parts that refer to gender:

Freundin (female) → Freund (male)
liebste (feminine) → liebster (masculine)

So you get:

Du bist mein liebster Freund. – You are my dearest (male) friend.

Notice:

mein (not meine) because Freund is masculine nominative.
liebster to match masculine Freund.

How do you pronounce liebste and Freundin correctly?

Approximate pronunciations (IPA + English hints):

liebste – /ˈliːpstə/
- lieb = lee
  - p
- st here sounds like st in stop (not like sht)
- final -e is a short, unstressed uh sound
  → roughly: LEEP-stuh
Freundin – /ˈfʁɔʏndɪn/
- Fr: like fr in free, but r is more guttural
- eu: pronounced like English oi in oil
- nd: like nd in and
- final -in: like in in in, but short
  → roughly: FROIND-in (with a German r and short final syllable)

Is there a more neutral way to say this, without sounding so emotional?

Yes. More neutral options:

Du bist meine Freundin. – You are my (female) friend.
Du bist eine gute Freundin. – You are a good (female) friend.
Du bist eine sehr gute Freundin. – You are a very good (female) friend.

All of these are less emotionally intense than meine liebste Freundin, which sounds very affectionate or intimate.