Learn Hardcore French: À mon avis, cette ville est magnifique.

à is a preposition meaning roughly “to/at/in.” a (no accent) is the verb avoir in the 3rd person singular (“has”), as in il a. At the start of a sentence, the capital should keep the accent: À, not A.

Could I say Selon moi or Pour moi instead of À mon avis?

Yes, but the tone varies:

Selon moi: neutral/slightly formal.
Pour moi: often “for me” (preference), and can sound more subjective; it’s fine in speech but not always ideal in careful writing.
D’après moi is also common. À mon avis is the safest general choice for “in my opinion.”

Why mon and not ma in mon avis?

Because avis is masculine, so the possessive is mon. Note: you also use mon before a feminine noun starting with a vowel sound for euphony (e.g., mon amie), but here avis is masculine anyway.

Is à mon opinion correct?

No. It’s not idiomatic. Say à mon avis, selon moi, de mon point de vue, or start with je pense que… You can say mon opinion est que…, but not à mon opinion.

Why is there a comma after À mon avis? Can it move?

It’s an introductory phrase, and French normally sets it off with a comma. You can also move it:

Cette ville est magnifique, à mon avis.
Cette ville, à mon avis, est magnifique.

Why is it cette ville and not ce ville or cet ville?

Because ville is feminine singular. Demonstratives agree:

Masculine before consonant: ce (ce parc)
Masculine before vowel or mute h: cet (cet arbre, cet homme)
Feminine: cette (cette ville)
Plural (both genders): ces (ces villes, ces parcs)

Does cette mean “this” or “that”?

Both. Context decides. To force the distinction:

cette ville‑ci = this city
cette ville‑là = that city

Could I say la ville instead of cette ville?

Yes, but it changes the nuance. La ville = “the city” (already known/assumed). Cette ville points to a particular city you’re indicating as this/that one.

Why elle/il est here and not c’est? Could I say C’est magnifique?

With a specific noun as subject, use il/elle est: Cette ville est magnifique.
Use c’est:

Before a noun with an article/possessive: C’est une ville magnifique.
For broad reactions or pointing to a situation: C’est magnifique !
Both are correct in their contexts.

Does magnifique change for gender/number?

Gender: no (same for masculine/feminine). Number: yes (add s in the plural).

Cette ville est magnifique.
Ce parc est magnifique.
Ces villes sont magnifiques.

Is très magnifique natural?

It sounds odd to most natives because magnifique is already very strong. Prefer vraiment/absolument/particulièrement magnifique, or use très beau/belle.

How does magnifique compare to beau/belle, superbe, splendide?

All are very positive:

beau/belle: common, broad “beautiful/nice.”
magnifique: stronger, “gorgeous/stunning.”
superbe: very positive, a touch formal.
splendide: very strong, sometimes literary.

How do I pronounce the whole sentence?

[a mɔ̃n‿avi, sɛt vil ɛ maɲifik]
Tips:

À = [a].
mon avis has a liaison: [mɔ̃n‿avi].
cette = [sɛt]; ville = [vil].
est = [ɛ] (final t silent here).
gn in magnifique = [ɲ] (like Spanish ñ).

Where are the liaisons?

Standard: mon‿avis (you pronounce the linking n).
No liaison in est magnifique (next word starts with consonant). Before a vowel you do link: est‿incroyable [ɛt‿ɛ̃kʁwajabl].

How do I make it negative?

Wrap the verb with ne … pas (elision before a vowel sound):
À mon avis, cette ville n’est pas magnifique.

Does à contract here like au/aux?

No. Contractions occur with definite articles: à + le → au, à + les → aux. With a possessive it does not contract: à mon avis (not “au mon avis”).

Is ville always feminine, and how do I refer back to it?

Yes: la ville, une ville, cette ville. Refer back with elle: Elle est magnifique. For several cities: Elles sont magnifiques.