Tengo buena memoria para las palabras nuevas, pero no para las fechas.

Because Spanish uses tener (to have) for this idea: tener buena memoria = to have a good memory.

A native English speaker might expect something like I am good at remembering..., but in Spanish the most natural structure here is with tener:

• Tengo buena memoria. = I have a good memory.
• No tengo buena memoria para los nombres. = I’m not good at remembering names.

So tengo is not about possession in a literal sense only; it is just the normal Spanish way to express this ability.

Because in Spanish, after tener, it is very common to leave out the indefinite article with qualities or characteristics:

• tener paciencia
• tener suerte
• tener miedo
• tener buena memoria

So tengo buena memoria sounds very natural.

You can sometimes say tengo una buena memoria, but it usually sounds more emphatic or more specific. In a general statement like this, tengo buena memoria is more idiomatic.

Because memoria is a feminine singular noun, so the adjective must agree with it:

• memoria → feminine singular
• buena → feminine singular

That is why you get buena memoria.

Also:

• bueno is the basic masculine singular form
• buen is the shortened form used before a masculine singular noun, as in buen libro

Since memoria is feminine, neither bueno nor buen fits here.

In Spanish, adjectives can go before or after the noun, but the position often affects what sounds natural.

With buena memoria, putting buena before the noun is the normal, fixed-sounding expression.
Memoria buena would sound unusual here.

More generally:

• many descriptive adjectives often come after the noun
• some common evaluative adjectives, like bueno/a, often appear before the noun in set expressions

So buena memoria is just the usual way to say it.

Because tener memoria para + noun means to be good at remembering a certain kind of thing.

Here:

• memoria para las palabras nuevas = memory for new words
• memoria para las fechas = memory for dates

It tells us the area or type of thing the memory is good or bad for.

Compare:

• Tengo buena memoria. = I have a good memory.
• Tengo buena memoria para los nombres. = I have a good memory for names.

Because Spanish often uses the definite article when speaking about things in general.

So:

• las palabras nuevas = new words, as a general category
• las fechas = dates, in general

To an English speaker, this can feel strange because English often leaves the article out:

• I’m good at remembering new words
• but Spanish prefers las palabras nuevas

Without the article, the phrase may sound less natural in this sentence.

Because in Spanish, most descriptive adjectives usually come after the noun.

So:

• palabras nuevas = new words
• libro interesante = interesting book
• casa grande = big house

This is one of the big differences from English, where adjectives usually come before the noun.

Also notice agreement:

• palabras is feminine plural
• nuevas is feminine plural

Because the second part is an ellipsis: Spanish leaves out the repeated words, but keeps the structure that is still needed.

Full idea:

• Tengo buena memoria para las palabras nuevas, pero no tengo buena memoria para las fechas.

To avoid repetition, Spanish drops tengo buena memoria, but keeps para las fechas:

• ..., pero no para las fechas.

That repeated para is necessary because it belongs to the phrase introducing what kind of things are hard to remember.

Because las fechas is not the direct object of tengo here.

The structure is:

• tener buena memoria para algo

So the important link is para + noun. That is why the shortened second half is:

• pero no para las fechas

If you said pero no las fechas, it would not match the structure properly and would sound incomplete or wrong in this context.

No. Here fechas means calendar dates:

• birthdays
• anniversaries
• historical dates
• the date of an event

For a romantic date or an appointment, Spanish usually uses cita:

• Tengo una cita. = I have a date / appointment.

So in this sentence, las fechas definitely means dates on the calendar.

Yes, it is very natural and correct.

The sentence joins two contrasting parts:

• Tengo buena memoria para las palabras nuevas
• pero no para las fechas

The comma helps mark that contrast clearly. In normal writing, Spanish usually uses a comma before pero when it connects two clauses like this.

Yes. The fully expanded version would be:

Tengo buena memoria para las palabras nuevas, pero no tengo buena memoria para las fechas.

That is grammatically correct, but it sounds repetitive. Spanish, like English, often leaves out repeated material when it is obvious.

So the shorter version:

Tengo buena memoria para las palabras nuevas, pero no para las fechas.

is more natural.