Mi profesora dijo que una buena reseña no cuenta todo el final ni explica todo el desenlace.

Profesora is the feminine form of profesor, so it tells you the teacher is female.

• mi profesor = my male teacher
• mi profesora = my female teacher

Notice that mi does not change for gender in the singular:

• mi profesor
• mi profesora

It only changes in the plural:

• mis profesores
• mis profesoras

Dijo is the preterite form of decir and means said.

• decir = to say / to tell
• dijo = he/she said

It is used because the sentence refers to a completed past action: the teacher said something at a specific moment.

Compare:

• Mi profesora dijo... = My teacher said...
• Mi profesora decía... = My teacher used to say... / was saying...

So dijo fits a one-time, completed statement.

After verbs like decir, Spanish often uses que to introduce what someone said.

• Mi profesora dijo que... = My teacher said that...

This works very much like English that, although in English that is often omitted:

• She said that a good review...
• She said a good review...

In Spanish, que is normally kept here.

Both are possible in Spanish, but the position of the adjective can slightly change the feel.

• una buena reseña usually means a good review in a general, evaluative sense.
• una reseña buena can sound more contrastive or literal, as if you are distinguishing it from bad reviews.

In many common expressions, adjectives like bueno/a often go before the noun when giving a general opinion.

So una buena reseña is the most natural wording here.

Reseña usually means review, especially a written or spoken evaluation of a book, film, series, play, etc.

In Spain, reseña is a very normal word for this idea.

Depending on context, it can also mean a short summary or brief notice, but here review is clearly the intended sense.

Contar has more than one meaning. Two very common ones are:

• to count numbers
• to tell or relate information, a story, or events

Here, no cuenta todo el final uses contar in the sense of telling or giving away.

Examples:

• Cuenta hasta diez. = Count to ten.
• Me contó la historia. = He/She told me the story.

So in this sentence, cuenta means something like tells or reveals.

Ni means nor or not even, depending on context.

In this sentence, no... ni... links two negative ideas:

• no cuenta todo el final ni explica todo el desenlace
• doesn’t tell the whole ending nor explain the whole outcome

This is a very common Spanish structure:

• No come carne ni pescado. = He/She eats neither meat nor fish.
• No llamó ni escribió. = He/She neither called nor wrote.

So ni is the natural way to say nor / neither after a negative verb.

Todo el final means the whole ending or all of the ending, not just the ending in a general sense.

Compare:

• el final = the ending
• todo el final = the entire ending / all of the ending

The speaker is emphasizing that a good review should not reveal all of it.

The same idea appears in:

• todo el desenlace = the whole outcome/resolution

They are related, but not identical.

• final = the ending, the final part
• desenlace = the resolution, outcome, or the way the plot gets resolved

So:

• final focuses on the last part itself
• desenlace focuses more on how everything is tied up or resolved

In reviews of books, films, or series, desenlace is a very useful word because it refers to the plot resolution, not just the last scene or last moment.

In Spanish, when a verb is conjugated, you do not use the infinitive with to as in English.

• explicar = to explain
• explica = he/she explains

Here the subject is una buena reseña, so the verbs are in the third person singular:

• una buena reseña no cuenta...
• una buena reseña no explica...

So explica agrees with reseña.

Spanish often avoids repeating the subject when it is already clear.

The subject una buena reseña applies to both verbs:

• una buena reseña no cuenta... ni explica...

English can do something similar:

• A good review doesn’t reveal the whole ending or explain the whole resolution.

Repeating the subject would sound unnecessary:

• una buena reseña no cuenta... ni una buena reseña explica...

That would be unnatural here.

Spanish often uses the definite article more than English does.

Here:

• el final = the ending
• el desenlace = the resolution/outcome

This sounds natural because the sentence refers to the ending/resolution of the work being reviewed. Spanish usually treats these as specific, understood parts of the story.

English might also say the ending, so this matches quite well.

Because the sentence reports a statement as a fact, not as doubt, emotion, desire, or uncertainty.

• Mi profesora dijo que una buena reseña no cuenta...
  = My teacher said that a good review does not reveal...

This is straightforward reported speech, so the indicative is used:

• cuenta
• explica

The subjunctive would appear in different kinds of contexts, for example after expressions of doubt, desire, or purpose.

Yes, in everyday modern Spanish, especially informal speech, people often talk about spoilers.

For example:

• Una buena reseña no hace spoilers.
• Una buena reseña no te destripa el final.

But the original sentence is more neutral and formal:

• no cuenta todo el final
• ni explica todo el desenlace

That makes it sound suitable for a teacher, classroom, or more careful written Spanish.

Not exactly a fixed expression, but it is a very natural combination.

Spanish often uses contar with parts of a story:

• contar el final = tell the ending
• contar toda la historia = tell the whole story
• contar qué pasa = tell what happens

So no cuenta todo el final simply means doesn’t tell the whole ending or doesn’t give all of it away. It is standard, natural Spanish.