In my opinion, credit cards is so useful (are very useful. - cards is plural so use "are" instead of they. You could use either "so" or "very". "Very" sounds more natural.). But, it has (they have - you are referring to the plural, "cards" which is "they") disadvantages, too. I have two reasons. First, you only have credit cards, you can pay for money. (This sentence is a little confusing. Do you want to say, "First, you can use credit cards for money." ?) Especially, when you haven't enough money. I think it is helpful. Second, it has a possibility of leaking your private information from credit cards. If security cord is leaked, your money are spent by unknown people. (If the security code is leaked, your money can be spent by unknown people.) I'm so scared, so I don't still use it. (... so I still don't use them. - Them refers to cards, which is plural. The word "still" would normally go before the verb.) Great start on your writing. I've fixed some things in() with explanations after the - . Most of your errors are in the use of pronouns (they, them, it ) and the verb, to be (am, is, are). This grammar is difficult for most people. Hang in there.