So here’s a quick recap of the ‘Rules for adding the suffixes–er, -ing, -ed, -est and –y.’ VO:Rule 1 is that for any word that ends in two consonants you just add the suffix. Words like ...

Sometimes they also change the original word's spelling. When adding a suffix you might have to double the last letter. For example, when adding -ed to 'drop', you need to double the 'p'.