"Religion is personal."
Translation:Η θρησκεία είναι προσωπική.
Why sometimes you must use a article and sometimes its wrong? I don't get it!
Usually, if a noun is the subject of the sentence, it will need an article, either definite or indefinite.
When a noun is the object of a sentence, it will always take a definite article, should the definite be appropriate in that context. Where it gets a little hazy is when an indefinite would be appropriate. We can, and usually do, drop the indefinite where there can be only one of something, or where there could be no confusion over quantity, or when we're making a general statement.
For example, if it's a hot day and you feel like an ice cream, you'd say Θέλω παγωτό. You're just making a general statement. But when you get to the ice cream shop, you'd tell the server that Θέλω ένα παγωτό, as you could conceivably want two or three ice creams, and you need to specify a specific quantity. Similarly, if you wanted to say "England has an economy", you'd just say "Η αγγλία έχει οικονομία", as England only has one economy.