Alguien vs. alguno
In Spanish, both alguien and alguno can be translated as "someone" or "somebody". While they are similar, they are not interchangeable. The main difference is whether you are referring to a general, unspecified person or one person from a specific, identifiable group.
Alguien
A1This is an indefinite pronoun that refers to an unspecified person in a general sense. It is the most common and direct translation for "someone" or "somebody". Use alguien when you don't have a specific group of people in mind. It is invariable, meaning it does not change for gender.
Alguien dejó su paraguas en la entrada.
(Someone left their umbrella at the entrance.)
¿Hay alguien que pueda ayudarme con estas cajas?
(Is there somebody who can help me with these boxes?)
Escuché que alguien me llamaba desde la otra calle.
(I heard someone calling me from the other street.)
Si alguien sabe la respuesta, por favor levante la mano.
(If someone knows the answer, please raise your hand.)
Necesitamos contratar a alguien con experiencia en marketing.
(We need to hire someone with experience in marketing.)
Alguno
A2This pronoun also means "someone" or "somebody," but it specifically singles out one person from a defined group that is either mentioned or clearly understood from the context. It essentially means "any one of them" or "one of them". Unlike alguien, this word has gender forms: alguno for masculine and alguna for feminine.
De todos los estudiantes, ¿alguno quiere ser voluntario?
(Of all the students, does any one want to volunteer?)
Hablé con tus hermanas. ¿Vendrá alguna a la cena?
(I spoke with your sisters. Will any one (of them) come to dinner?)
El entrenador miró a los jugadores y preguntó si alguno estaba cansado.
(The coach looked at the players and asked if any one of them was tired.)
Hay varios expertos en la sala. Seguro que alguno puede resolver tu duda.
(There are several experts in the room. Surely one of them can answer your question.)
¿Alguno de vosotros tiene un bolígrafo que me preste?
(Does any one of you have a pen I can borrow?)
Summary
In short, use alguien to mean "someone" in a general, undefined way. Use alguno or alguna to mean "someone" or "any one" when you are referring to a person from a specific, known group of people.