I'm studying single and double replacement, and I was wondering how you know if a element is a anion or a cation. I already know anion is a negatively charged ion, and a cation is a positively charged ion , but I don't understand how you know which one's are positive and negative?How do you know which one to replace if you don't know which one is the cation or the anion?

Elements tend to form a single type of ions; either cations or anions.

One way to 'learn' them is through practice.

Another way is to know the position of that element in the periodic table. The elements after Group III tend to be cations and elements up to Group III tend to be cations.