Which of these can refer to a boy but not to a girl?

a) cantante
b) bailarin
c) atleta
d) joven

in which case would you use "saber"?

a) to say that you know san salvador
b) to say that you know maria
c) to say that you have met your friend's mother
d) to say that you know how to play the piano

In Spanish (Castilian) there are three genders for nouns: male, female and neutral. Except in the case of people and animals, the grammar gender is only a convention that indicates the article and the ending of the adjective that are used with a specific name.

Now, bailarín (dancer) is a singular masculine noun and its feminine form is bailarina. Therefore the word bailarín cannot be used for both genres.

Nevertheless, the nouns cantante (singer), atleta (athlete) and joven (young) are neutral and can be used for both genres in Spanish.

The verb “to know” has two principal meanings in Spanish:

“saber” or “conocer”  

-It is used to indicate knowledge of a fact.

-It is used when we want to express the ability to do something or talk about knowing skills (also learned skills). For example: driving or playing piano; writing the verb in the following form:

Note that often the verb saber is followed by the words cómo (how), que (that), qué (what), quién (who), dónde (where), cuándo (when), cuál (which), por qué (why).

- It is used when we have had some experience with a known thing or a known person or to indicate that you have a relationship with a person.

-It is also used to say that we have been or visited a place.

-It may also mean "to meet" a place, a thing or a person (for the first time).

Now, in this sentence we are talking about a skill: to play piano, therefore we must use saber:

I know how to play the piano

The other sentences match with conocer:

-If you want to say that you know San Salvador (that you have been or visited a place) the correct translation is:

Yo conozco San Salvador

-If you want to say that you know Maria (that you have a relationship with a person) the correct translation is:

Yo conozco a María

-If you want to say that you have met your friend's mother (that you have met a person for the first time), the correct translation is:

Yo he conocido a los amigos de mi madre