Who can tell me specifically how to write chemical formulas?

Writing the chemical formulas of simple substances: Except for hydrogen, oxygen, chlorine, nitrogen, fluorine, iodine, bromine, etc., which need to add 2 in the lower right corner of the chemical symbol, the chemical formulas of other simple substances are the chemical symbols themselves. (For junior high schools)

Steps for writing chemical formulas for compounds containing two elements:

① Write positive valence elements on the left and negative valence elements on the right;

② Mark the valence of the corresponding element and find the least common multiple of the absolute value of the valence;

③ Find the number of corresponding atoms and write it in Arabic numerals in the lower right corner of the element symbol (with brackets around the atomic group).

What you need to remember is the main valencies of common elements and atomic groups. You can memorize it through formulas. There are many online. Then just do more, practice more and memorize more. The more you write, the easier it will become. Junior high school chemistry mainly focuses on memorization.