SOME!
Voy a tomar un café con unos compañeros de clase alemanes.
Voy a tomar un café con algunos compañeros de clase alemanes.
What’s the difference?
Unos / unas or algunos / algunas? In English this is easy – we have just one word – SOME!
Obviously we have the masculine & feminine versions, which are driven by the gender of the noun – but how do we decide whether to use UNOS or ALGUNOS?
Like many things in Spanish, it’s the difference between specific & vague – or what my students will know I call ‘wishywashy’.
Unos is vague, algunos is more specific, with a sense of exclusion.
Sometimes,
either can be used. If we want to say, ‘Some of the …’, the we can say ‘Unos de
los …’ or ‘Algunos de los …’.
Estos son ALGUNOS DE LOS mejores momentos de tu vida.
Estos son UNOS DE LOS mejores momentos de tu vida.
The first has a sense of specificity – a specific ‘some’, probably with some common theme.

Otherwise, they aren’t interchangeable.
We use ‘unos’ without ‘de’ when there’s no clear quantity, nor any comparison.
In the following sentence there’s no indication of any comparison, nor of how many.
La escuela de idiomas va a introducir UNOS cursos nuevos.

When we have a comparison, either direct or implied, but we aren’t using ‘de’, we use ‘algunos’.
Here, there’s a clear comparison:
¿Por qué saben tocar la guitarra ALGUNAS personas, pero otras no?

But here, it’s implied that some of the students will receive prizes, but others won’t.
El colegio planea dar premios a ALGUNOS alumnos.

This shows both together:
Hay ALGUNOS mariscos que no me gustan, pero hoy voy a comer UNAS gambas.

So back to the question at the beginning…
Voy a tomar un café con unos compañeros de clase alemanes.
Voy a tomar un café con algunos compañeros de clase alemanes.
What’s the difference?
The version with ‘unos’ is vague – wishywashy – & means that I’m going to have coffee with some classmates who happen to be German. It might be all of the classmates who are German, or only some of them.
The version with ‘algunos’ implies that I’m going to have coffee with some German classmates – but definitely not all of the German classmates.