Mi profesora dice que falta un paréntesis en esa definición.

In modern Spanish, possessive adjectives like mi, tu, su, nuestro normally go directly before the noun, without an article:

• mi profesora = my teacher
• tu libro = your book

So la mi profesora is not standard modern Spanish here.

Also, profesora is feminine, so it means a female teacher. If the teacher were male, it would be mi profesor.

Spanish nouns often show grammatical gender, and for many words referring to people, the ending changes depending on whether the person is male or female:

• el profesor = the male teacher
• la profesora = the female teacher

So mi profesora tells you the speaker’s teacher is female.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

• dice = he/she/it says

Since mi profesora is already stated, adding ella would usually be unnecessary:

• Mi profesora dice... = natural
• Mi profesora ella dice... = not natural here

You could say Ella dice que... if you wanted extra emphasis or contrast, but normally Spanish prefers to omit the pronoun.

After verbs like decir (to say), Spanish usually uses que to introduce the reported statement:

• Dice que falta un paréntesis = She says that a parenthesis is missing

This que is very common in Spanish. English can sometimes drop that:

• She says that...
• She says...

But in Spanish, que is normally kept:

• Mi profesora dice que...

Here, un paréntesis is the grammatical subject, even though it comes after the verb.

So the structure is basically:

• falta un paréntesis = a parenthesis is missing

Because un paréntesis is singular, the verb is singular:

• falta

If it were plural, you would use faltan:

• Faltan dos paréntesis = Two parentheses are missing

A very common learner mistake is to think the verb should agree with an unstated there, like in English there is/there are. But in Spanish, the verb agrees with the thing that is missing.

In this sentence, faltar is best understood as to be missing.

• Falta un paréntesis = A parenthesis is missing

This is a very common Spanish pattern. The thing that is absent becomes the subject:

• Falta azúcar = There is no sugar / Sugar is missing
• Faltan páginas = Pages are missing

English often uses there is/are missing or is missing, while Spanish often uses faltar directly.

Yes. Paréntesis is masculine:

• el paréntesis
• un paréntesis

So you use masculine articles and adjectives with it.

Also, paréntesis is one of those nouns ending in -s that can still be singular:

• un paréntesis = singular
• dos paréntesis = plural

The article usually tells you whether it is singular or plural.

Yes. The singular and plural are the same in spelling:

• un paréntesis
• dos paréntesis
• varios paréntesis

So you identify singular or plural from the article, number, and verb:

• Falta un paréntesis = singular
• Faltan dos paréntesis = plural

The written accent shows the stress falls on the syllable té:

• pa-rén-te-sis

Without the accent, Spanish spelling rules would suggest a different stress pattern. So the accent is necessary.

This is useful both for pronunciation and spelling, since paréntesis is an esdrújula-type word, and those always carry a written accent in Spanish.

Because the idea is that the missing parenthesis is in that definition.

• en esa definición = in that definition

If you said de esa definición, it would suggest a different relationship, more like of that definition, which is not what you usually want here.

So:

• falta un paréntesis en esa definición = a parenthesis is missing in that definition

Esa means that, referring to something not especially close to the speaker.

Basic contrast:

• esta definición = this definition
• esa definición = that definition

In Spain, people may also distinguish:

• esta = near me
• esa = near you / not near me
• aquella = over there, farther away

In many everyday situations, though, esa is simply the normal choice for that.

Because definición is a feminine noun:

• la definición
• esa definición

Demonstratives must agree in gender and number with the noun they describe:

• ese libro = that book
• esa definición = that definition

So the feminine singular form esa is required.

Yes, but those mean slightly different things or are structured differently.

• Mi profesora dice que falta un paréntesis...
  = My teacher says that a parenthesis is missing...
  This is neutral reported speech.

• Mi profesora dice: falta un paréntesis...
  = My teacher says: a parenthesis is missing...
  This introduces the exact words more directly.

• A mi profesora le parece que falta un paréntesis...
  = My teacher thinks it seems a parenthesis is missing...
  This sounds more like an opinion or impression than a straightforward statement.

So the original sentence is the most natural way to report what the teacher says.

The original word order is very natural:

• Mi profesora dice que falta un paréntesis en esa definición.

Spanish often places the verb before the subject with faltar:

• Falta un paréntesis rather than
• Un paréntesis falta

The second is grammatically possible in some contexts, but it sounds marked or unnatural here.

So the usual, idiomatic order is:

1. Mi profesora dice
2. que
3. falta un paréntesis
4. en esa definición

Not naturally in English.

A more literal grammatical comparison might tempt you to think:

• falta = lacks

But the natural English meaning is:

• a parenthesis is missing or
• there is a missing parenthesis

So when you see faltar in this kind of sentence, it is usually better to think in English as to be missing, not simply to lack.