If somebody speaks that to you, how do you tell if it means 'The man eats rice' or 'The man is travelling'?
It never means "The man is travelling". German doesn't have a continuous aspect. Both "The man travels" and "The man is travelling" translate as "Der Mann reist".
The man is travelling = Der Mann reist or Der Mann ist auf Reise.
why doesn't 'reis' have der/die/das?
in this case the man is eating rice in general not a specific rice. You can also say Er isst Brot, er isst Kartoffeln=he eats potatoes
How do we can understand a noun is femmenine or sth else?