Learn Hardcore French: Quelqu’un frappe à la porte; j’y vais. - Someone is knocking at the door; I am going.

Yes—Quelqu’un is a single word meaning someone/somebody. It historically comes from quelque + un, but it is written together with an apostrophe: quelqu’un. It is invariable (no plural) and takes a singular verb.

Is Quelqu’un masculine? Do we ever say quelqu’une?

By default, quelqu’un is grammatically masculine. You can see quelqu’une (feminine) when you want to emphasize the person is female, but it’s much less common. The verb still stays in the third person singular either way.

Why is it frappe à la porte and not frappe la porte or frappe sur la porte?

frapper à la porte = to knock at/on the door (idiomatic for knocking).
frapper la porte = to hit/strike the door (not the usual way to say knock).
frapper sur la porte = to bang/strike on the door (more physical/forceful). For a normal knock, use à.

Why à la and not au?

Because porte is feminine. à + la stays à la. You would use au only with masculine singular nouns (à + le → au).

Why is it la porte (the door) and not une porte (a door)?

French typically uses the definite article for context-shared, obvious items, like the door of the place you’re in. So la porte is the natural choice when someone knocks where you are.

Does frappe mean both is knocking and knocks?

Yes. French simple present covers both English simple present and present progressive. Quelqu’un frappe can mean someone is knocking right now.

What does j’y vais mean here, and what does y refer to?

Literally it’s I’m going there. y is a pronoun that stands for a place or an à + thing/place complement. Here it refers to the door (to that spot), and the whole phrase idiomatically means I’ll get it / I’m coming.

Could I say Je vais à la porte or just Je vais instead of J’y vais?

Je vais à la porte is correct but longer and less idiomatic in this context.
Just Je vais sounds incomplete.
Common alternatives: J’y vais ! or J’arrive ! (very natural for I’ll get it).

Where does y go with different tenses and in negatives?

Before the conjugated verb: Je n’y vais pas.
With compound tenses: before the auxiliary: J’y suis allé.
Near future: Je vais y aller.
Imperative: after the verb and hyphenated: Vas-y !; negative imperative: N’y va pas.

Can y replace a person (à + person)?

No. y does not replace people. Use indirect object pronouns for persons:

Je parle à Marc → Je lui parle, not J’y parle.

Why is it j’y vais and not je y vais?

Elision: je becomes j’ before a vowel sound. y starts with a vowel sound [i], so you must write j’y.

Could I say Je vais y?

Not by itself. You either say J’y vais (pronoun before the verb) or, if there’s an infinitive, Je vais y aller. French object pronouns normally come before the conjugated verb, except in affirmative commands (e.g., Vas-y !).

Can I replace Quelqu’un frappe with On frappe?

Yes. On frappe à la porte is very common and natural to mean someone’s knocking (an indefinite subject), similar to English someone/they’re knocking.

How would I say There’s someone at the door?

Il y a quelqu’un à la porte. That reports a presence. Quelqu’un frappe à la porte specifically mentions the action of knocking.

Is the semicolon necessary? Could I use other punctuation?

A semicolon is fine and shows a close link. You could also write a period, a comma (in informal writing), or even a dash: Quelqu’un frappe à la porte — j’y vais.

Pronunciation tips for the whole sentence?

Quelqu’un ≈ kel-kun (the final n nasalizes the vowel; no audible n).
frappe à ≈ frap-a (clear p; link smoothly to à).
la porte ≈ la port (t is pronounced).
j’y vais ≈ zhee veh (the j+y gives a [ʒi] sound; final s in vais is silent).

Does frapper always mean knock?

No. frapper means to strike/hit. With à + a door (or window), it means to knock. Without à, it often means hit:

frapper quelqu’un = to hit someone
frapper à la porte = to knock at the door

Are there other verbs for knocking?

Yes:

toquer (à la porte): informal, very common in speech.
taper (à la porte): to bang/knock (can feel a bit stronger). Neutral and standard is frapper à la porte.