Cette voisine est une amie de Marie.

Because cette is the feminine singular form of the demonstrative adjective.

French changes this/that depending on the gender and number of the noun:

• ce
  • masculine singular noun beginning with a consonant
• cet
  • masculine singular noun beginning with a vowel or silent h
• cette
  • feminine singular noun
• ces
  • plural noun

Here, voisine is feminine singular, so cette voisine is correct.

Because voisine is the feminine form of neighbor.

• un voisin = a male neighbor
• une voisine = a female neighbor

So this sentence tells you that the neighbor being talked about is a woman.

In modern French, cette can mean either this or that. Context usually makes it clear.

If French wants to be extra specific, it can add:

• -ci for this
• -là for that

For example:

• cette voisine-ci = this neighbor
• cette voisine-là = that neighbor

But in everyday speech, just cette voisine is very common.

Because amie is the feminine form of friend.

• un ami = a male friend
• une amie = a female friend

So the sentence says that the neighbor is a female friend of Marie.

Important: the gender of amie refers to the friend, not to Marie.
If the friend were male, you would say un ami de Marie.

French often uses de to show relationships such as possession, family connection, or friendship.

So:

• une amie de Marie = a friend of Marie / a friend of Marie’s

This is the normal French structure here. A native English speaker may expect something closer to Marie’s friend, but French usually expresses that with de.

Because Marie is a proper name, and proper names normally do not take an article in standard French in this kind of sentence.

So you say:

• de Marie

not:

• de la Marie

This is the standard pattern with names after de.

The article changes the meaning:

• une amie de Marie = a friend of Marie’s, one among possibly several
• l’amie de Marie = Marie’s friend, the specific one already known in context

So une is indefinite, while la / l’ is definite.

Usually, in this sentence, une is needed because amie is being used as a noun meaning a friend.

• Cette voisine est une amie de Marie = This neighbor is a friend of Marie’s.

Without the article, amie sounds less like a regular count noun and more like part of an expression meaning friendly with / friends with, which is a bit different in tone and structure.

For a learner, the safest normal form here is:

• est une amie de Marie

A careful pronunciation is approximately:

set vwa-zeen eh tun ah-mee duh mah-ree

A more IPA-style version is:

[sɛt vwazin ɛ tyn ami də maʁi]

A couple of useful notes:

• cette sounds like set
• voisine sounds roughly like vwa-zeen
• amie has two syllables: a-mie
• there is a liaison between est and une, so it sounds like eh-tune

Yes: the structure is very straightforward French word order:

• Cette voisine = subject
• est = verb
• une amie de Marie = complement

So it follows the normal pattern:

subject + verb + noun phrase

That makes this a very typical and useful beginner sentence pattern.