Alii otium longum malunt, sed pater dicit meliorem esse modum vitae cum labore et requie.

Alii means others or some others.

It is the nominative plural masculine form of alius, alia, aliud (other, another). Here it is being used by itself as a substantive, so it means other people rather than modifying a noun.

So:

• Alii ... malunt = Others prefer ...

This is very common in Latin: an adjective can stand alone and imply a noun like homines (people).

Both otium and longum are in the accusative singular neuter.

That is because malunt (they prefer) takes a direct object: the thing being preferred.

So:

• otium = leisure, rest
• longum = long
• otium longum = long leisure

The adjective longum agrees with otium in:

• case: accusative
• number: singular
• gender: neuter