"The ones at the table are brothers."
Translation:Cei de la masă sunt frați.
There was an another sentence meant the same: "Cei de la masa sunt frati". So, does that mean I can use both "aceia" and "cei" for this?
Trying to understand (thinking out loud, please feel free to contradict): So, the easy bit is that a masculine plural form is required.
Attempting to address the hard bit, the word "ceia" seems to be a demontrative pronoun (rather than a demonstrative adjective) because (hoping I have understood this) it is followed by an adjectival phrase (which has qualified it, rather than it qualifying something else) and then it becomes the subject (rather than the object) of the verb (here, "sunt").
On the other hand, "aceia" is the (admittedly masculine plural) form of the demonstrative adjective (rather than pronoun) that is used when it follows (rather than precedes) the noun.
The choice of preceding or following the noun by the demonstrative adjective depends on which of the following is to be emphasised: the particular noun itself; or the particular subset of the items denoted by the noun (which is merely acting as as class label for the whole of the set) that has been identified by the demonstrative adjective.
Except that it seems that "aceia" can sometimes be used as a demonstrative pronoun.
Arghhh! My brain is melting!