Relative Pron

The antecedent is simply the noun you are describing. In "The man who left," the 'man' is the antecedent. In Arabic, allaḏī must agree with this word.

Arabic grammar considers the description of an indefinite noun to be like a direct adjective. Just as you say baytun kabīrun (a big house), you say baytun yaqau hunā (a house [that] sits here). The connection is implied.

man means "who" or "the one who" generally, often without a preceding noun (e.g., "I saw who did it"). allaḏī links to a specific noun (e.g., "I saw the man who did it").

Look at the case of the noun. If it ends in -āni (subject), use allaḏāni. If it ends in -ayni (object/preposition), use allaḏayni. Match the sound!

No. mā is used for non-human things meaning "that which" or "what" (e.g., "I saw what happened"). It usually doesn't refer back to a specific noun mentioned earlier.

You use allātī or allawātī. Both are correct, though allātī is more common in modern standard media.

In Modern Standard Arabic (MSA), absolutely yes. It's a marker of good education. In dialects, rules are much looser and often use illi for everything.

This is advanced! You use allaḏī + a noun with a pronoun suffix. Example: ar-rajul allaḏī sayyāratu-hu ḥamrā' (The man who his-car is red).

Rarely in the way you think. It usually follows a noun. If you want to say "He who studies succeeds," you would use man (man yadrus yanjaḥ).

It is called Al-'A'id (الْعَائِد). It 'returns' the meaning back to the original noun.

Yes. Singular animals use allaḏī (masc) or allatī (fem). Plural animals use allatī (because they are non-human plurals).

The sentence will feel "hanging" to an Arab listener. "The book that I read..." (and then what?). The -hu completes the thought loop.

Not really in formal writing. You must spell them out. allaḏī has a shadda on the Lam that is often not written but always pronounced.

Strictly speaking, no. allaḏīna is masculine plural. However, if the group is mixed (men and women), masculine wins, so you use allaḏīna.

It is a "sun letter" assimilation if preceded by Al-, but these words are special. Just pronounce a strong L: al-ladhī.

It sounds very foreign. Like saying "A man the which I saw." It breaks the flow of the indefinite state.

Yes, allatāni (nominative) and allatayni (accusative/genitive).

Yes! "The house in which I live" = al-bayt allaḏī askunu fī-hi. Note the fī-hi (in it) at the end.

Spelling rules have evolved, but standard modern spelling is allaḏī. Always stick to the standard form to be safe.

Remembering to DROP the word for indefinite nouns. It feels wrong to English speakers to just say "I saw a man [gap] was running," but that is correct Arabic!