Learn Hardcore Spanish (Latin America): Mi novia es muy sincera y me da un beso cuando llego a casa. - My girlfriend is very sincere and gives me a kiss when I get home.

Short (unstressed) possessives that go before a noun: mi, tu, su, nuestro, su, etc.
- mi novia = my girlfriend
Long (stressed) possessives that usually go after the noun or stand alone: mío, tuyo, suyo, nuestro, etc.
- la novia mía = the girlfriend of mine
- la mía = mine

You cannot put the long form directly before the noun, so mía novia is incorrect. For my girlfriend you normally say mi novia.

Does novia always mean “girlfriend”? Can it also mean “fiancée” or “bride”?

Novia can mean:

girlfriend
fiancée (a woman you’re engaged to)
bride (on the wedding day, e.g. la novia y el novio = “the bride and groom”)

Context tells you which one it is. In everyday Latin American Spanish, if nothing else is said, novia usually means girlfriend. The masculine form is novio (boyfriend / fiancé / groom).

Why is it sincera and not sincero?

Adjectives in Spanish must agree in gender and number with the noun:

novia is feminine singular → the adjective must also be feminine singular.
sincero (masc.) → sincera (fem.).

So:

Mi novio es muy sincero. (boyfriend)
Mi novia es muy sincera. (girlfriend)

Why is it es muy sincera and not está muy sincera?

Ser and estar both translate as to be, but:

ser is used for inherent or lasting traits (character, identity).
estar is used for temporary states or conditions.

Being sincera (sincere / honest) is described as a character trait, so ser is used:

Mi novia es muy sincera. = My girlfriend is very sincere (by nature).

Está sincera would sound unusual; it would suggest she is being sincere right now, as a temporary state, and even then most speakers would phrase that differently (e.g. está siendo sincera).

Why does sincera go after novia instead of before, like in English?

In Spanish, most descriptive adjectives normally go after the noun:

una casa grande = a big house
una persona inteligente = an intelligent person
una novia sincera = a sincere girlfriend

Some adjectives can go before the noun for stylistic or meaning changes, but sincera is most naturally placed after: una novia muy sincera.

What exactly does me da un beso mean? Why not just me besa?

Literally:

me = to me
da = gives
un beso = a kiss

So me da un beso = she gives me a kiss.

Differences:

dar un beso (a alguien) is very common and a bit more neutral / affectionate:
- Mi novia me da un beso. = My girlfriend gives me a kiss.
besar (a alguien) focuses more on the act of kissing:
- Mi novia me besa. = My girlfriend kisses me.

Both are correct; in everyday speech dar un beso is extremely common for this type of sentence.

In me da un beso, what is the function of me?

Me is an indirect object pronoun, meaning “to me” or “for me”.

The structure is:

ella (subject, implied)
me (indirect object = to me)
da (verb “gives”)
un beso (direct object = a kiss)

So the literal structure is: (She) gives a kiss to me. → Ella me da un beso.

Can I say me da besos instead of me da un beso?

Yes, but the meaning changes slightly:

me da un beso = she gives me a kiss (one kiss, or a single affectionate gesture)
me da besos = she gives me kisses (more than one, or habitually)

Context will decide which sounds more natural. For a single welcome kiss when arriving home, me da un beso is very typical.

Why is it cuando llego a casa and not cuando llego en casa?

With verbs of movement (arrive, go, come), Spanish uses a (“to”) before the destination:

llegar a = to arrive at / to
ir a = to go to

So:

llego a casa = I arrive home / I get home

En casa means “at home” (location), not movement:

Estoy en casa. = I am at home.
Trabajo en casa. = I work at home.

So you say llego a casa, not llego en casa.

Why is it a casa and not a la casa or a mi casa?

Spanish often uses a casa (without article or possessive) to mean the general idea of “(to) home”:

Llego a casa. = I get home.
Voy a casa. = I’m going home.

You can say a mi casa or a la casa de mis padres when you want to specify whose house:

Llego a mi casa a las seis. = I arrive at my house at six.
Voy a la casa de Ana. = I’m going to Ana’s house.

But when it’s just “home” in general (your own home in context), a casa is the natural choice.

Why is it cuando llego (present) and not cuando llegaré (future), even if it’s about the future?

Spanish normally uses the present tense after cuando when talking about future time:

Te llamo cuando llegue. = I’ll call you when I arrive.
Me da un beso cuando llego a casa. = She gives me a kiss when I get home.

The idea of the future is understood from the context or from the main verb, so you don’t say cuando llegaré in this type of sentence. You keep llego / llegue in the present form.

Should it be cuando llego or cuando llegue? When do I use the subjunctive here?

Both forms exist, but they mean different things:

Cuando llego a casa (present indicative)
- Talks about habitual actions, things that regularly happen.
- Mi novia me da un beso cuando llego a casa.
  = She gives me a kiss when I (normally) get home.
Cuando llegue a casa (present subjunctive)
- Refers to a specific future event that hasn’t happened yet.
- Mi novia me va a dar un beso cuando llegue a casa.
  = My girlfriend is going to give me a kiss when I get home (later).

In your sentence, it’s describing a routine, so cuando llego (indicative) is correct.