Can they be used synonymously? What is the difference in meaning and use between the two?
They are not always interchangeable. In these two sentences taken from the lesson, they are interchangeable:
Necesito un bolígrafo. ¿Tienes alguno/uno?
I need a pen. Do you have one?
A Melisa le hace falta una camisa roja. ¿Le prestas alguna/una?
Melisa needs a red shirt. Can you lend her one?
However in the examples with the plurals "algunos/algunas" we wouldn't use the plural "unos/unas":
-Fuimos a ver museos. -Muy bien, y ¿pudisteis visitar algunos? (not "unos")
-We went to see museums. -Very good, and did you manage to visit any?
Tengo muchas postales de Venecia, ¿quieres algunas? (not "unas")
I have many postcards from Venice, do you want some?
Also, when we refer with alguno/alguna to "the odd one/ a random one" you cannot use "uno/una", because then it won't mean "the odd one/a random one":
¿Probaste muchos platos exóticos en el evento gastronómico? -Sí, probé alguno.
-Did you try lots of exotic dishes in the gastronomic event? -Yes, I tried some (random dishes).
¿Probaste muchos platos exóticos en el evento gastronómico? -Sí, probé uno.
-Did you try lots of exotic dishes in the gastronomic event? -Yes, I tried ONE.
It would mean something different in this case.
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