So sometimes you might want to know when to add -er or -est to a word when you're comparing objects, and I'm going to show you how. I have three books here. One is a dictionary, one is Best American Poetry Anthology, and one is a slim book called A Simple Heart. So say that I want to compare how heavy these books are. Well, I might say that the dictionary is light compared to a bowling ball. I might say that the poetry anthology is lighter. In that case, I'm adding er to the end of the adjective light to show that this one is lighter than that one.
Say I want to say that A Simple Heart is lightest, meaning out of all of them, it weighs the least. And in that case, I simply add est to the adjective light to get lightest. One trick that you should know is that if you are using an adjective that has more than one syllable, you can't always add er or est to the end of the word when you're comparing. Instead, you use the words more and most. So if I want to let you know about which one of these books is more interesting than the others, this is how I would do it. I would say the dictionary is interesting.
I like grammar, so the dictionary is interesting. The poetry anthology is more interesting, and A Simple Heart is the most interesting. In this case, interesting has four syllables, so that's why we use the words more and most when we're making a comparison. This hopefully clears up when to add -er or -est to an adjective when you're comparing items and when you want to use the words more and most instead.